Here's another aftermarket that's cheaper than the mitocorps one ($200). Also, you don't have worry about hookin it up to the rear backup lights.
This is probably the one I'll be getting.

http://www.donnellye.com/_private/prod01.htm

Also here's an Xterra owner who has given a description of how they installed it!

http://www.donnellye.com/customer.htm