Tinted windows are nice. More tint the better.
Some sort of air mattress, depending on the vehicle. Even just a closed-cell pad could help even a surface out.
Reflextix - cut to fit the windows, with leftover under your sleeping pad if in a pickup or van (or Honda Element, such as I own)http://www.homedepot.com/Reflectix/h_d1/...catalogId=10053Skreenz (or a bunch of mosquito netting you can secure)http://www.skreenz.com/Something for
self-defense. I camp with my dog so don't worry about being surprised.
If you don't have access to a bathroom - something to pee in and
handi-wipes for freshening up a bit.
Headlamp (Petzl e+lite is all I need to read)
Chemical hand and feet warmers (for all but hot summer nights).
Keep the key fob nearby. I have a "PANIC" button on mine.
Crack a window or sunroof to keep condensation from building up.
Many Wal-Marts allow overnight stays.