I have this mounted in all of my vehicles:
http://gopro.com/
There are several options for recording in a vehicle but this is the one I finally chose. I've used two other types of cameras over the past 6 years and the GoPro is the best so far in my opinion. For vehicle's you don't need the most expensive camera setup and shop on Amazon to get the best price. I've been using the GoPro for a year I think...maybe a bit longer. My camera has over 500 hours of use on it and works perfectly.
I did opt for the best GoPro camera and opted to get the largest capacity SD card (32GIG) which gives me just over 8 hours of continuous recording. I use a Motorola V195 cell phone power cable to power the camera from the vehicle. The camera is very small, lightweight and when mounted in a "Skeleton" case (needed for car mounting and best audio pickup) is almost unseen. Many people that do see it think it is a radar detector. Unplugged the camera will run about two hours on it's battery.
My video/audio recordings have come in handy in the past. When using my previous recording system I was broadsided by an inattentive driver. He got a ticket and when his insurance company was asked to pay they turned it around and said it was my fault. Well, the video/audio recording was then sent to my insurance company. When they saw it and heard the guy say "I was turned around talking to my daughter"...in her car seat behind him when he ran the stop sign...well, his insurance company paid every penny.
Several of us also carry discrete recording devices just in case we run into a rogue cop or a rogue citizen who causes a scene. If you would like a list of devices some of us carry please PM me. I'll send you a link of what I carry.