I'd like to be able to patch my portable MP3 player through my car
stereo, but don't want to get all fancy with separate amplifier. I've
tried the FM transmitter approach but the one I bought had kinda
crappy sound and the tuning drifted all over the place with
temperature changes.

My auto is a Subaru Forester 99 and currently has a fairly generic
looking Clarion factory AM/FM/cassette system. Looked at some kits
that patch into CD changer port but that doesn't seem possible here.