how do the rear park sensors work?

They are basically a SONAR system. They send out a high-frequency audio signal then listen for the echo several times a second and activate the alert when an echo returns within a preset amount of time, which means it bounced off an object within the "safety zone" behind the vehicle.

Thank you for the reply. Any idea where I can get a wiring diagram ? From what I have seen on the prices for the sensors (4) it looks like the rear view mirror is a better deal

Any auto parts store should have or be able to order in a owners repair manual such as Haynes or Chilton. The nice thing about rear park sensors is they cover an area that you can't see in the rearview mirrors.

Have the Haynes manual. There is no mention of the RSS. The Ford Manual which is 2 manuals cost about $70 to $100 (I also have a 2000 Lincoln TC. The 3 manuals cost $100.). Between the cost of the manuals and sensors it could be cheaper to put in a video backup system.

Yeah, I would do that rather than spend that much on repair manuals.