If you have a clear view to a rear window from the front, I'd suggest getting one of those stick-on window lenses, the largest one you can find (and see from the driver's seat). They provide a pretty decent portion of the utility of a rear view camera and are quite inexpensive and highly reliable (and wireless). At night they're not as useful since you can't get a good idea of the lane markings etc. in the lens, making it hard to figure out where things are in relation to you, but during the day they're handy, even if one has a camera.