MP3 player that fits in your ashtray
A wired FM modulator
A cable with dual-RCA to 3.5mm stereo plug
Whatever is needed to remove your radio and install the FM modulator (probably a screwdriver, wire cutter/strippers, electrical tape)
Step 1: Install an FM Modulator
My 97 Ford Escort is weird, and required a special tool made from a coathanger to remove the radio. Since all cars are different, I won't discuss how to install the modulator. Generally speaking, though, you'll probably need a screwdriver, some wire cutters/strippers, and electrical tape. It may or may not be easy.
Step 2: Pull a Stereo Cable from the Modulator through the Ashtray
Step 3: Modify Ashtray
Step 4: Plug in the MP3 Player and Put the Ashtray Back In
The player I used (irock ezmp3) uses sd cards for memory. Given the minimalist interface, it'll be difficult to manage a lot of music on a single card. I plan to save music on multiple small-memory sd cards, maybe with a few albums each, and just keep the cards in a Sucrets tin.
Using an FM modulator to listen to a $20 mp3 player is definitely low-end. It sounds pretty good, though.