The iTrip, or any other FM radio transmitter-type iPod adapter is NOT a long term or very reliable solution. Basically, they all suck. A boatload of iTrips are returned to the Apple Stores every day because mainly they offer an inconsistent and unreliable operation, at best. Cassette adapters are actually much better than the FM adapters, and cheaper, of you have a cassette player obviously.
If you want iPod in your car or anywhere else, just get the right equipment if you can afford it.
In the car, use a car adapter that plugs directly into an input/aux adapter (Google "iPod car adapters"), or get a new stereo system the works with the iPod, like the Alpine or Pioneer systems.
If you really enjoy your music and have a huge selection, spend the money and get a nice stereo or adapter that works with your current factory stereo. You spent $200 to $400 on your iPod, might as well do it right.
A good site for adapters that work with factory stereos is:
www.logjamelectronics.com
I bought an adapter there for my previous vehicle which had a factory CD player but no changer, so the adapter plugged into the port for the changer. The adapter was hidden behind the dash and it worked flawlessly. My new setup is the Alpine head unit with iPod Adapter. Navigation is slow, but the system kicks major arse!