This is one heck of an easy and useful hack with a cell phone handsfree headset so you could hook it up to your car's aux imput. I have the SE K750i that plays mp3 with a 1GB memory stick duo pro. I hacked the headset so I could plug the phone into my car's auxillary input to play my mp3 like an iPod which I don't need anymore. If I get a phone call while the music is playing, the music will stop and automatically switch over to voice call. The voice comes out of the car speakers and I just talk normal. The mic in the handsfree picks up my voice just fine.
Tools:
Pocket knife
Soldering iron and solder
wire stripper
Parts you need:
1\8" female jack extention cable from Radio Shack or your local electronic store. ($6.00)
SE K750i handsfree headset.

01. Use a knife to crack open the handsfree mic assembly.

02. Remove the front and back cover and set them aside.

03. De-solder the right ear piece from the solder point on the PCB.

04. Cut the 1\8" femail jack with about 6" of wire and strip 1" of the outter insulation. You'll see a black, red and ground wire.

05. Strip a little insulation off of the red and black wire and tint the ends with solder.

06. Solder the red wire to the +S and ground to the -S solder point on the PCB.

07. Solder the black wire to the hanging black wire to the other end of the mic PCB. You have to remove the heat shrink first.

08. Now tape up the bare solder of the black wire with electrical tape. Keep is thin or else it won't fit back into the covers.

09. You're done with the hack. Put the mic PCB back into the covers you removed in step 02.

10. Once the covers are re-installed, cut the left ear piece off.

12. Here is the completed mod.

Now mount the phone on the dashboard, plug the hands free into the phone and the other end to the car's auxillary input.

The sound quality is just as good as any MP3 players. I love the voice calling because the I don't need the headset in my ear.