But I guess I don't understand how.

If you point a camera out one window, what happens when *something* from a direction the camcorder is not pointing?

Ideally, you would have 4 cameras - one pointing out each side. A 4-channel surveillance DVR in the trunk might work - along with 4 small security cams. There are also single channel mobile recorders available.

http://www.mobilewitness.com is one source.

As for mounting, since this is not very common (in "consumer passenger" vehicles), you will need to make mounting systems - unless you are willing to have custom installations done like law enforcement or public transit. Given your budget, I suspect this is not the case. We don't know what vehicle you are using, what type of "view" you want to cover, the resolution of the video required...