What year is your vehicle?

The 2001-2008 had bumper mounted sensors and as previously stated the repairer should have returned the vehicle with the sensors functioning.

The earler 1996-2000 models had parking sensors fitted adjacent either side of the number plate and if I recall correctly the wiring followed that of the rear wipers through the tailgate pillar into the head lining before going down the inside of the rear right panel where they were earthed....so that might be a good starting point.

Hope this helps