Strange but true...the cassette player in one of my cars actually ejects the "fake tape" adapter. It works great in the other car, however, and it's a permanent fixture
Another alternative is an FM transmitter. I've included a couple of links. Maximum transmission range is about two meters -- so if your antenna is in the rear, this could be a problem.
Here are some of the available models:
irock
Audiax
Arkon