here is a delco 5 band EQ cassette deck that I wired up with an aux input. I also wired up a factory radio button to switch the signal between the radio and aux input. 3.5mm cable is wired out the rear of the deck and ran through center console for a cleaner look. No 3.5mm jack and cable coming out the face of the deck to clutter things up.

EQ, LOUD button, and volume, fade/balance all work.

With this style radio, I would swap out the "search" button for the tape deck for the aux input button. Non search tape decks, I would swap out the DNR button.

http://www.youtube.com/watch?v=rAlVsK-UFKg