My wife and I found a pretty good looking old radio in the thrift store a while back and picked it up with the intention of adding a simple aux input for her laptop, until we remembered she had a spare little can style Bluetooth speakers. I decided to give it a shot and try to fit it inside the radio.
At first I thought about tapping into the radio's own amplifier, but I couldn't find the schematic for this particular radio (at least not for free), and I couldn't be bothered to trace out the circuitry, so I tried just connecting the speaker output wires to the actual speaker in the radio. Turns out it's loud and clear enough, so I just went with that.
The next challenge was the power; the original BT speaker was battery powered, charged with a USB cable, so the obvious option would have been to add a 5v power supply somewhere and connect it to the charging port. I didn't have one handy, but started probing around and found that the light bulb that illuminates the scale has 6V AC across it. I was able to source some decent additional current without drooping too much, and those wires were switched by the volume knob in the radio.
I ended up just removing the battery and adding a small regulator circuit instead, so that the Bluetooth board could be switched from the volume key. I could't find a LM317 in my junk box so I whipped up a circuit using a TL431 from a broken switchmode power supply:
And assembled it on perfboard:
The output is exactly 4.2V, which is the voltage of a fully charged Lithium-Ion battery. All thrown together, we tested with our phone and the audio quality was pretty good (for a bluetooth speaker) and the range is also surprisingly good.
The modification was the least invasive possible. All the original circuitry of the radio is still operational and can be put back to work by just swapping the speaker cables and disconnecting the regulator board from the light bulb.