Back up warning sensors

I have a 99 model that has the backup sensors on the bumper and a switch on the counsil. But, of coarse it dont work. I have the ford service manual for the 99's but it doesnt speak of the system being on this vehical. I have no way to trouble shot the problem. I have checked both fuse boxes and the is no fuse labled for this system. I have also inspected the sensors wiring and cant see any visible defects. Any help out there