Ok, so if I’ve got this right... You want to put in a CarPC type of monitor and just use the composite video jacks until you actually get your CarPC right? If so then...
http://store.mp3car.com/Lilliput_8_W..._p/mon-029.htm
this is the monitor I have. The first one died in short order but mp3car RMA'd it and the 2nd one has worked like a champ. The size is really nice, the color won't knock your socks off, but it's not half bad, the resolution is good, the touchscreen is, IMO, perfect, you don't have to press it hard at all, and it's done great for me anywhere from 20deg to 120deg. This monitor works for everything I need it to. That being said, it's not at all bright in direct sun, but that may be fine for you if it's going in the back seat and you have dark tint.
I thought about getting a 10.4 inch, but I couldn't quite fit it. Still, a good thread:
http://www.mp3car.com/vbulletin/lcd-...lcd-215-a.html
Other than that just look around the forum, there are lots of people with lots of opinions on almost every possible monitor. (though if you were a bit more detailed on exactly what you were looking for in terms of size, brightness, how you plan to use it, etc, it would help everyone to give you better suggestions)
--------------------
When it comes to headphones I suggest you stay away from anything built into the monitor. If you want to do it right either get a stand-alone wireless headphone setup, or get a bluetooth adapter for your CarPC and a set of stereo headphones (that's the setup I have)
For the bluetooth setup you'll probably want to go with the Motorola HT820 headsets (search them on eBay) and for the bluetooth adapter I find that the ASUS I have sounds great with the Motorola HT820's
http://www.newegg.com/Product/Produc...82E16833320016
Well that's my 2 cents.