There's three problems with using the GCT as an in-car player.
1.) The 10" screen is huge, relative to most vehicles' dashboards, and it's hard to find a place to mount it. It also screams "steal me", especially if it's just screwed down in its factory case.
2.) The LCD isn't designed to withstand the plastic-melting temperatures car interiors reach in the summer, nor the sub-freezing temperatures reached in the winter. Expect a very short lifetime. It's also not nearly bright enough to be used outside during the day.
3.) The sound sucks, especially when amplifed. This can be overcome with an Extigy, but an Extigy is another $150 and takes up even more space.
I bought my GCTP for the specific purpose of building a car MP3 deck, but reality quickly set in. I might stick it in one of my TR8s at some point, just for the funk factor, but since I don't drive them that often, pulling it out isn't too much of a hassle.
It can be set up in 30 seconds if you've got a 12V-19V adapter (or a power inverter - probably the better choice, as it protects you from the starter.) and an AUX input on your head unit, but it sounds poor, and requires a lot of TLC to avoid frying or freezing it.
Mine's in my garage right now, hooked up to an old amp. So at least I've got it *near* my cars. :)