I tried the Belkin one. It sucked ass. Too bad to hear the iTrip isn't any better but I'm not surprised. They need to run more signal to be any use.

For most car stereos, you can get an adapter that converts the CD changer input into a pair of RCA inputs. Run an RCA->1/8" cable out to the front of the dash and you're golden. That's what I did, and I've recently taken to leaving the dock in the car, plugged in. The dock happens to fit really well in a VW cup holder, so you can always see the iPod display, and fiddle without holding it.