instead of pulling the rca to the dash... you can mount a female headphone port to the dash somewhere and then use a male-male wire to connect between the port and your mp3 player/etc.

I thought about that, and decided against on the grounds that it introduces another junction -- the extension cable connecting to the stereo, then the male-male cable connecting to the extension, then the device connecting to the male-male cable. The way I'm thinking of doing it, the extension cable plugs right into the device, plus the extension cable can be removed easily for other purposes if need be.

Also, RCA-male to 1/8" female cables seem to be rare. At any rate, Radio Shack doesn't list them on their site and Best Buy doesn't seem to have them either.

Maybe I'll stop by my local car audio shack and see what they have to say...