The problem with the tuneAir is that you don't get to use your telephone if your car doesn't have bluetooth. I personally use something like the following. It requires power from a USB adapter in the cigarette lighter (not the built-in USB port) and also plugs into the AUX port. So...
I would recommend buying a high quality 1/8" stereo patch cord (yes, this is the same kind of plug you find on 99% of portable audio devices/headphones) and running that from inside your radio to your alternative sound source. I'm using a 6ft gold plated patch cable from radio shack....