I use a standard 4 channel home security recorder and mount a couple of standard home security cameras. Usually at least one over the rear view mirror, facing forward, and one facing to the rear.
The DVR, like most others, is 12V so it is trivial to wire into the system so it comes on automatically. With a 500MB drive, it keeps about two weeks of recordings.
The whole setup runs under 200 USD, 100USD for the DVR and about 35USD each camera.
I haven't caught anything exciting, but I did recently "shame" a local law enforcement officer who accused me of not fully stopping. I simply pointed to the camera and he told me to move along.