Hey!
I've used a WIRED FM modulator to transmit audio from Axim to car audio system.
My product is similar to this one:
http://cgi.ebay.com/DIGITAL-STEREO-F...QQcmdZViewItem
Car's antenna is connected to the modulator and the modulator's output to the head unit's antenna connector. The audio signal is inserted into the modulator using two RCA connectors, so you need a 3.5mm jack to RCA cable.
I had to place a ground loop isolator between the axim and the modulator to cut off noises that charging the axim through a lighter adapted caused.
With these components the sound quality is :approve: for me. There are no interferences, as the audio signal is transmitted through the antenna's wire and not using the air. I found this the best solution when your head unit hasn't got an AUX input available (most of them I'm afraid) IMHO.
Maybe I can post a diagram if you need it.