We bought a Griffin iTrip for the PT Cruiser and it works suprisingly well.
We have an auxillary port in the other car so we're kind of spoiled - the iPod really sounds best when you have a direct connection. We tried various FM transmitters and all worked well but there was always some sort of static - this one really works great.
You plug it into your cars cigaratte lighter so it also charges your iPod. It doesn't go through the CD player - it goes through the radio.
If you go to this page
http://www.griffintechnology.com/products/itrip/ we have the one on the far left (black).
I bought ours at BJ's but I'm sure you can get it anywhere iPod accessories are sold.