My brother's car had that problem a while back with his friend's Grand Cherokee. Either he or his friend fixed it, I think, but I can't remember how. I'll try and hit him up and see what he says.
By the way, is there a way to get better bass on my new headphones? They're pretty nice but I like my tunes to have big, powerful bass just below the distortion zone, which my last pair (now borken; the new pair was a gift from the friend who broke the old ones) provided perfectly. Turning the EQ to Bass Booster on my iPod works okay (it distorts some songs' bass, though), but doesn't quite cut the mustard. What should I do?