What/which interface did/are you using?  I have looked at the radio transmitter and the cassette tape ones.  I found the two bogus to say the least.  I had to return them.  Ebay sells a few interfaces too, but as you know, their return policy is not as generous as the stores.  So my best bet so far is to get an interface installed behind the dash by the dealership.  A friend of mine did it before he drove the car across the country.  Cost is ~$500.

Any other suggestions?