I wrote before on the ideal car dock for an MP3 player but the truth is we could use something even simpler sooner. On my recent trip, we brought the cassette adapter but there was no tape player in the rental car. We forgot the FM transmitter, but that's not as good anyway.
So right away let's see a small headphone plug on the car stereos to do a nice aux input, especially if you are taking away the tape. Duh.