Our 2007 Hyundai Sonata has XM radio, which we never use. Unfortunately, it has no line-in jack for our phones. After dealing with ineffective radio tuners for a few years, I finally did some research on how to add a line in jack to the radio. It turns out a few people had done it before, and it was pretty straight-forward, as long as you're comfortable removing the radio and doing some light soldering to the XM harness. The final result is a (sort of) working line-in jack that can plug right into any audio player with a 1/8" jack.
Unfortunately, it doesn't quite work right -- it seems like the vocals only come out of the right speaker, and I lose all sound from the right speaker when I plug the phone into a charger. I haven't been able to figure out why this is happening.