my '05 did that too... they worked, ONLY occasionally. When you put the car in reverse, you would hear them blip for just a second, then the dash display would say off, instead of on.
I fixed mine by buying a new Expedition, now they work great. Alright, so I'm only helpful with part... my '05 did do same thing, yes. I just never got around to fixing as I actually look over my shoulder and check my surroundings before backing. However; before trading in, I went to dealer to have fixed, and they quoted me some ridiculous amount to fix. So then I took to an independant shop, said it was an easy fix, just needed to replace one or all of the sensors.
So, if you're handy, can probably do yourself...