This is a really simple way to add Bluetooth audio to basically whatever you want, for $8AUD or less!
Step 1: Acquire Bluetooth Audio Adaptor.
I got mine from ebay, it cost $8AUD, but I could have gotten one straight from China for $4, I didn't want to wait. There are plenty of types available, but mine looked like this. It is powered by USB, And has the standard 3.5mm stereo jack on the other end.
Step 2: Wire It Up!
What you'll need;
A stereo, speaker, iPod dock, to which you would like to add Bluetooth to.
Your bluetooth audio adaptor
Hot glue gun
A donor female USB plug
A donor 3.5mm stereo male plug (my BT adaptor came with a cord)
Also helpful, wire cutters
Step 3: Wiring - Power
First we need to find power for the adaptor. Opening up the top of the dock I found this small board. Being an iPod dock I suspected there would be a 5v supply at the connector just as USB (and iPod charging) requires. And it was even marked! A quick check with the multimeter confirmed it.
Cutting a spare lead I had laying around I found red, black, green, and white wires, the latter are for data, so they can be removed, or taped aside. I checked the board markings against the plug and found they were the red and black wires, easy! I stripped a little insulation back from each and soldered the wires in, taping each.
Step 4: Wiring - Audio
For the audio input I wanted to keep it all internal, and still have the use of the RCA inputs if needed.
I opened the back cover by 6 screws, and removed the amplifier board to access the RCA inputs. Using the multimeter, I found the left, right and common ground and soldered some spare speaker wire straight to the board, securing it with plenty of hot glue.
I then punched a small hole in the dividing panel inside the dock, and ran the wires through.
Next I soldered the speaker wires to the donor 3.5mm plug that came with my adaptor. When cut, you will see 3 wires, googling "3.5mm pinout" provided the info I needed to get it right first time.
I then taped them all up, and secured it all nicely with a bit of hot glue, including the hole I made for the wires.
Step 5: Reassembly, and Finishing Touches.
I reassembled the amplifier board into the dock, and plugged in the adaptor, that's pretty much it!
I stuck on a NFC sticker to the inside of the lid so I can use it to turn on my phones bluetooth.
Testing it out, it worked first time, there is a little bit of interference, but this dock always had some to start with.
For $8 and about 90 minutes, I'd say I done well!
Step 6: Update
I felt that I wanted the comfort of the blue flashing light on the adaptor. So I decided to mount it on the inside cover with the LED shining through a small hole I drilled. To do this I had to pop it out of the case and remove the USB plug, as it was getting in the way.
At this point I realise just how small this thing is, without the USB and stereo plugs, just 15mm by 30mm and maybe 2-3mm thin. Could be great for some bluetooth headphones.... Maybe a quad copter battery or two..... Feel free to run with that, if it hasn't been done before, I'm sure someone will now.
I also played around with the range, and yes, as someone said below, it is terrible, maybe 3 meters streaming from my phone. I also found after some use that if I'm using my phone, it tends to cut out badly when I am using wifi, which doesn't happen when I stream to other devices. So I suggest finding a better adaptor, I'm sure there's plenty out there.