Are you after a reverse alarm or a parking sensor?
The sensor is the ultrasonic aid that stops you hitting things when reversing, where the alarm is to warn people you are reversing.
If you just want to warn people, I found This
Previous Ride = 2011 JGC CRD Overland
Current Ride = 2014 Silver JGC SRT
with Pano, HK and Spider monkeys.
Mods = CLSTech Ribbon Mod
Best 1/4 13.4@102Mph