Note: There's a video of the finished project at the bottom of this page.

I've seen many DIY projects that turn an old vintage radio smart, but I've never seen one that doesn't remove at least some parts of the original radio. I've decided to try and let both of them co-exist - allowing me to have a smart device disguised as a vintage radio, but also, well, enjoy a perfectly functional vintage radio.

This project aims to allow both the original FM radio and the smart device (in my specific case Amazon Echo Dot, but you can choose others) co-exist and play simultaneously.

This is my first ever project posted here, and it's also been a first for me in many other ways (soldering etc), so take this into account when you read.

Supplies

My radio has only one speaker, so I didn't care about stereo speakers and whatever they entail. If you intend to have multiple speakers, adjust the supplies list accordingly, and adjust my guide to fit your needs.

A vintage FM radio: I've chosen the Sony TFM-9450W - I love the aesthetics and it was relatively affordable. Whatever you choose, make sure there's enough clearance for everything.
A smart device: Something such as an Amazon Echo Dot/Google Nest Mini/etc. The one caveat is that it MUST have an 3.5mm aux out. I'm already using Alexa for my smart home, and had an Echo Dot 4th Generation, which has an aux out, so I didn't even bother with understanding how to connect to its mainboard circuit to transfer audio. If you're going with an Echo Dot, it's preferable to use the one without the LED clock display, unless you can find something to do with it (you cannot detach it from the main board as it won't boot without it).
An Audio Signal Mixer - It takes 2x 3.5mm as inputs and has a 3.5mm output.
A power adapter for your signal mixer - Make sure it fits in terms of connecter and voltage/amps.
2x 2RCA to 3.5mm cables - You cut the RCA ends and connect one of the cables to the speaker, and the other to the original inputs that were connected to the speaker. Consider getting an additional cable or two, to have as reserve in case you screw up (I accidently ruined one cable during soldering).
1x Male to Male 3.5mm extension cable - Will connect your smart device to the signal mixer.
Heat Shrink Wire Connectors: To connect the radio audio output cable to the 3.5mm cable.
(optional) Restor-A-Finish: If your radio is wooden and old, consider restoring some if its old finish. Make sure you choose the right color.
(optional) A Voltage Up/Down Converter: If the radio operates at a different voltage range than the one in your country, you may need a Voltage Up/Down Converter. Make sure it can support the wattage of the radio you chose. Since the radio I bought operates at 110V, I had to buy an up converter, ended up with a 26VA 110V -> 220V converter.

Note:

After completing my build, I realized that I could have picked more suitable parts - namely, instead of the 2RCA to 3.5mm cables, I could have just gone with this 3.5mm 2Pole Male Cable.

This cable makes it easier to wire everything, and solves one discomfort:

No need to fiddle with the 2RCA cable which is fragile and has groundings to take care of.
No need to insert the radio board 3.5mm input partially due to it being a stereo plug connect to a singular speaker (more on that later).
This can be problematic if you can't set it properly in prevent it from moving - I had no such issue.

So to wrap this up, I'd definitely go with 2x3.5mm 2Pole Male Cable instead of the 2x 2RCA to 3.5mm cables if I had to do this again.

Step 1: Disconnect Speaker

Take a proper photo of everything for documentation - important when reattaching stuff and wondering about color coding/polarization etc.
Desolder the speaker connections.
Detach the cables and nip them.

Step 2: Connect Radio Audio Output to 3.55m Cable

To properly connect the cables, we need to take polarization and grounding into consideration, to reduce noise as much as possible.

From what I gathered (some reading + ChatGPT questions):

The yellow radio cable is positive end (+).
The black radio cable is the negative end (-).
The red RCA is the positive end (+).
The white RCA is the negative end (-).
The silver wire inside the RCA cable is for grounding.
Grounding goes to the negative end.

So, to properly connect, I had to do the following:

Strip the radio yellow/black wires.
Cut both RCA ends
Strip the external RCA wire (black)
Strip the external RCA wire (red/white)
Connect (twist) both the silver grounding cables, the RCA white and the radio black together
Connect (twist) the RCA red and the radio yellow together.
Use the heat shrink wire connectors to secure the connectors with a heat gun/hair dryer

Make sure you strip enough cable to have ample length to work with! The internal RCA cables are very thin and will tear easily.

Note: The attached photo is an intermediate one - I forgot to take one with the proper heat wire connectors, and now it's all tucked up nicely inside the radio and I don't wanna take it out.

Step 3: Solder 3.5mm Cable to Speaker

Now we take the other 2RCA cable and connect it to the speaker. Follow the same polarization/grounding logic mentioned in the previous step.

Cut both RCA ends.
Connect (twist) both groundings and the white RCA together.
Twist the red RCA to make it fit into the speaker connector easier (I even folded it so it's thicker).
[Before soldering, consider testing the connections - you can even solder while music is playing from the smart device (unplug the radio itself). Check the next step for testing instructions]
Solder the end with the groundings+white RCA to the speaker end that was initially connected to the black radio cable (negative).
Solder the end with the red RCA to the other end of the speaker (positive),

In the photo: My very first (and terrible) soldering job. I had to redo it, but didn't take a photo - it ended up looking much better (trust me).

Step 4: Test the Connections

Use the signal mixer to test the connections.

Connect speaker 3.5mm into the output channel of the mixer.
Connect radio 3.5mm into the first input channel of the mixer.
Note: As I mentioned in the supplies section, since the 3.5mm plug is a stereo one (3 pole) but is connected to a single speaker (mono), you have to insert it partially (about 75% in). Just fiddle with it until you can hear much louder than if you fully insert it.
Connect your smart device to the second input channel using a 3.5mm extension cable.
Start the radio, make sure you can hear it well.
Play something on your smart device, make sure you can hear it well.

Step 5: Disassemble Your Smart Device

Now it's time to disassemble the smart device. I used this guide to disassemble the Echo Dot 4th Generation Without Clock. Take your time - flex cables are very gentle - I know that because I tore one and bought a new Echo Dot the next day (weird flex, I know).

You want to be left with the minimal set of components that maintain functionality. In my case it was the mainboard and the mic board, connected by a flex cable.

Step 6: Fit the Audio Signal Mixer

I ended you screwing the audio signal mixer to the floor of the radio, while making sure the knobs do not protrude and are not sunk in.

Also make sure there's enough clearance to connect/disconnect cables from the signal mixer.

Ignore the Echo Dot mainboard screwed to the side of the radio - that was a temporary placement which turned out to be bad (more on than in a moment).

Step 7: Fit the Smart Device

I initially screwed the Echo Dot mainboard to the side of the radio, but this turned out to be a suboptimal placement:

The mainboard LEDs are facing the internals of the radio - no chance for them to be seen (the front of the radio is sealed shut but the backside has many holes which can allow light to escape.
The side that's facing the radio wall is going to suffer from heat issues.
There's no real place for the microphone - I intended to put it out of the radio, close the radio cover and then hang it on the cover - this way the microphones are in the open air, at the expense of risking the flex cable + having a circuit outside (ugly).

I then resorted to hanging the mainboard+mic board on the back cover using cable ties:

Low chance of causing damage to the device/flex cable.
LEDs face the holes, which create a very cool effect and allow to see when the LEDs are engaged.
Although mics are facing the internals of the radio, they are attached to the back panel which has holes and is very close to the walls, which reflect back the sound. I have no issues with voice.
Zero heat issues.
Slight disadvantage: Need to disconnect the Echo power/aux if/when it's required to open the back cover.
Slight disadvantage #2: Zip tie cables are seen from the back, I don't care since its facing the wall.

Side note: The Echo's CPU was touching the speaker through a thermal pad, to dissipate heat. I didn't have any small grills, but did have a thermal pad. If you have a small grill, you can attach it to the CPU using a thermal pad to further assist cooling, althought i doubt this is required.

Step 8: Cable Management + Closing

Cable management: Wasn't really an issue. I used some cable ties to tie some cables together, and used my fingers to tuck some other cables behind boards and that's it.

Back cover: I had to cut a hole for the signal mixer's knobs, as well as nip a corner for the Echo/signal mixer power cables.

Step 9: Sit Back and Enjoy Your New Smart Vintage FM Radio

I think it turned out pretty cool, don't you agree?

Note: I've set both of the signal mixer knobs to their maximum amplification power. You can always balance it differently/fiddle with it in real time, I just have no real use for it.

Introduction: Vintage Smart (Fully Intact) FM Radio