I've used a Peak wireless for over a year now in my full-sized truck.
I wired it to my backup lights so it only comes on when I put the truck in reverse. After a couple of rains the camera got water in it but I pried it open, dried it out, and sealed it with silicone cement - it has stayed dry ever since. Rain or morning mist will usually obscure the image. The image is good when the lighting
is just right, but often glare will wash out the scene to where it's practically useless.
I first put the transmitter in the space by the tail light
but the range must have been a bit too great since I got a tremendous amount of interference, so I moved it closer to the cab. I put the transmitter in double zip-lock bags and zip-tied it to the frame, stretching the cable to its length, to where it nearly reached the cab. I now get much less interference. In a small vehicle it should do even better, but still not great. It picks up interference from other vehicles, cameras at railroad crossings and stop lights, etc. Once our screen even showed the front of our own truck! The camera on the car in front of us broadcast to our unit - very interesting.
All-in-all it does a fair job at best. I got the Peak with the 3 1/2" screen, which I consider too small, so I can't imagine the 2 1/2" screen as being adequate. For Christmas I'm getting a hard-wired unit with a 7" screen. I'll report on it after I've used it for a couple of months.
This is the monitor I ordered from Amazon ($35.68): http://www.amazon.com/gp/product/B00...ls_o02_s00_i00
And this is the camera ($15.26):