Search and find out for your particular headunit, but it'll at least have component video in and rca stereo in for audio. The only problem is that your car pc won't look very good on the screen compared to on a screen made for higher resolutions. If you havent already - check out some of the screens people use in the show off forum...it could cost less than a kenwood/etc unit in the long run and a carpc can still play dvds and do radio (little sketchy here - should be solved fairly soon tho.)
Or since the new Mustang has a double-din slot you could put a flip up vga screen (check http://www.digitalww.com) in the first, and a regular headunit below it to keep easy access to radio and cds. There are plenty of ways to make a great setup though so do whatever you think would be best for you.