Disconnect and isolate the battery negative cable.Raise and support the vehicle.Locate and disconnect the engine wire harness connector from the backup lamp switch connector receptacle.Check for continuity between the two terminal pins in the backup lamp switch connector receptacle.With the gear selector lever in the REVERSE position, there should be continuity.With the gear selector lever in any position other than REVERSE, there should be no continuity.If the switch fails either of these two continuity tests, replace the ineffective backup lamp switch.