Most decks have an input on them, but they are less than optimum sound quality. Nearly any interface will degrade the quality.
What I run in both my truck and car is a deck that plays MP3/WMA. I can put 120-150 songs on one CD, and its excellent quality. I only paid a little over 200 each for the decks too. Probably the best solution, just burn the songs from your IPOD on to a 10 cent CD!