One of the questions we’ve been asked a lot recently is: How can I add an audio in jack to my existing car stereo? Some car stereos just don’t allow this capability, others have AUX IN pins in the back of the unit that can be used to solder on a jack, and some — where the CD players are separate from the receiver/radio (a.k.a. the “head unit”) — can be spliced into. This hack involves the latter set up. This guy has a 2001 Toyota Corolla where the CD player is in a separate unit. All he basically did was split the wires going from CD to the head and route them to a toggle switch and AUX IN jack (so he can switch between CD and auxiliary input). Presumably, this could be done with any stereo that has separate components like this wired to the head.
Tip: If you’re interested in knowing if your car stereo has AUX IN hackability, do a search on the car or stereo/head unit model and “aux in,” and with some pokin’ around, you’ll likely find what you’re looking for.