My experience on the LOW end- I've used very small LED panels (such as these http://www.filmtools.com/li56flpamolp.html) on suction cups mounted to the car, near the rear view mirrors, and gotten ok results. They were dimmed VERY low. Shooting on a T2i, 30mm 1.4 lens I believe. Terribly compressed YouTube image, but it's all I got at hand.
I've also used christmas lights plugged right into the car, you can get battery powered ones as well. Litepads also seem very viable, but I don't have enough experience with them to weigh in.