I asked about input devices last week, and found this wireless device on Amazon. I'm using it with Mediacar now and it works rather well as a complete replacement to a mouse or trackpad.
It's pretty simple... large circular pad for movement - quite accurate, and a left and right mouse button, and another toggle button to switch functionality of the mouse buttons to map to page-up and page-down.
Works nicely and shouldn't look out of place in most cars - its a dark/gunmetal grey with sliver buttons.
Now I just need to find something in mediacar that well let me map the right mouse button to 'next track' or something...
-= Supra Twin Turbo MK4
-= Lotus 7 replica
-= Escort MK1 with 2.0 16v XE
-= Xbox-Linux-in-car-system (v1)
-= EspressoPC, 256Mb, 40Gb, WinXP (v2)
-= Pioneer AVHP6400 6.5" headunit, MEHP9100 MD/CD, CDXP1230, Infinity Reference 605cs/6502i & 2x RA7502