Maybe this has already been in a former thread, but I don't remember seeing it.
I ordered a wireless backup alarm yesterday,(mod.yps-300M)
from 12voltshop.com for $99. plus shipping. It has 2 sensors that mount under the bumper and as most of the ones that I have
read about, the wires hook up in the back to the backup power, it has a beeper that beeps faster as you get closer,but what is intersting,it has a small readout unit that you can place on the dash,or wherever with no wires. It reads out in meters or feet. Their phone # 877-212-8050.