Step 1: Acquire Bluetooth Audio Adaptor.
Step 2: Wire It Up!
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
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
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 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
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.