I had this issue and ended up having to bring my truck to a shop to figure out what i was missing as the guy spent almost an entire day working on it since he did not know how to get it to work i do have an answer for you. there is an aditional power / activation wire that needs to be run to (not sure if its the same on all vehicles) a module above the pedals in your car (its a green wire in a plug). you will have to look up the wiring diagram for your vehicle.
Also if you are running off your reverse light you need up change the fuse in the pdc from the 5amp to a 10amp or youll blow it an
