I first tried this project with a bluetooth earpiece but was not pleased with the sound quality. I found a bluetooth stereo headset for cheap and thought I'd try the project again. Now I plug a bluetooth dongle in to the aux in in my car and stream Pandora through my stereo.

Step 1: Disassemble One of the Headphones

On my particular headset there were three screws, t5's to be specific. When I opened the earpiece up I found the main board. This board had the battery, bluetooth module, aux in and the mic. The black wire coming off the left of the picture runs through the headband and connects the secondary board.
This is something I didn't realize. I assumed when I started this project that there would be a wire connecting the other speaker but found that it connected to another board that operated the volume control and powered the speaker.

Step 2: Disconnect the Speaker

Disconnect the speaker wires from the speaker being careful to leave them attached to the board. I suppose you could solder directly to the board but that's just extra work. Also, this would be a good time to go ahead and disconnect the wire that connects the two headphones. Doing that will leave you with something looking similar to the picture.

Step 3: Finish the Disassembly

Once you remove the wire that connects the two headphones and disassemble the other headphone housing you end up with something that looks like the picture.

Step 4: Connect the Wires

Take a stereo 2.5 mm cable, cut one end and strip the wiring. Under the black sheath is a red wire, white wire and a bare wire. Split the bare wire and twist in to two seperate wires. Strip the red and white wire.
The bare wire serves as your ground. The red can be your stereo right positive and the white can be your stereo left positive.
On my headset there was a blue wire and a red wire. On the right headphone I soldered the bare wire to the blue and the red wire to the red wire. On the left headphone I soldered the bare wire to the blue wire and the white wire to the red wire.

Step 5: Reassemble the Housing

Before you start screwing things back together, connect the wire that connects the two headphones and pop out the speakers. The connecting wire can rolls up nicely in the spot once occupied by the speaker. The 2.5 mm wire comes out where the band that connected the two headphones used to connect.

Step 6: Connect the Two Pieces

The adhesive that once held the padding that covered the speaker does a nice job for me in holding the two headphone pieces together. I suppose you could glue them if you wanted.
Plug the jack in to the aux in in your car or run it through your tape deck, connect your phone and stream music through your car stereo.

Also, this particular headset had a mic included. My next step is to run the mic to a place in my car that can easily pick up my voice and have my phone calls through my stereo. I haven't gotten to that yet but let me know if you can get it to work.