park assist sensor

I currently drive a 2008 GMC Sierra with a broken back up sensor on the rear bumper, which causes me to get a "service park assist" message on my dashboard information center. I also have an aftermarket backup camera wired to a Kenwood DVD receiver. Does anybody know of a way to rewire or "trick" the computer into thinking that all of the sensors are working. If not, is there another source other than GM to get a new sensor. My preference would be the first option due to the fact that I use the camera to see behind me. Thanks for any help.

a quick search of fleabay netted a gm back up sensor for $18 from the states as well as countless others from chinaAdvanced

Thanks for the help dasbogie, I have called local junk yards and auto part stores. They all told me I would have to go to GM for a replacement with close to a $100 price. I'll definetaly check out your sources. I doubt the quality will be as good as the originals, but with the back up camera I use, my main goal is to just complete the circuit and loose the annoying service park assist message every time I jump in the truck. I'm ok with paying up to around $30 to get the audible alerts back in my truck, but no way is that sensor worth what GM wants for it. Thanks again