We have a very old iTrip from Griffin (not sure of the spelling). It seems to do the job ok. I have heard good things about their latest version, so that might be worth looking into.

I typically use an auxiliary input into my car radio, so I've moved away from that. Of course, the new(ish) car's aux input isn't working, soooo....

I am the Doctor, and THIS... is my SPOON!