I use the XA-300 to enable the Aux In.

It works really well, you don't need to have an auto-changer plugged into the other side, some of the others I saw when I was looking added an extra input but needed to have a changer plugged in to work. In fact I don't even pass audio through it.

It does make some clicking noises, but I only hear them because I have it under the drivers seat. It's a rip off at £100 (or $100!!!) but it works.

Cheers

Cris.