The back-up light switch is located on the left side of the transmission case. To check its functionality, turn the ignition key to the On position and move the shift lever to Reverse; the back-up lights should illuminate. If they do not, first check the back-up light fuse, and if it is blown, trace the circuit for a short-circuit condition. If the fuse is intact, raise the vehicle securely on jackstands and place the shifter in Reverse. While working under the vehicle, unplug the back-up light switch electrical connector and use an ohmmeter to check for continuity across the switch terminals; continuity should be present, and if not, the switch needs replacement. If continuity exists, check for voltage at the electrical connector; one terminal should have battery voltage with the ignition key in the On position. If no voltage is detected, trace the circuit between the fuse block and the electrical connector for an open circuit. If voltage is present, trace the back-up light circuit between the electrical connector and the back-up light bulbs for any open circuit issues, keeping in mind that both bulbs could potentially be burned out. For replacement, raise the vehicle securely on jackstands if not already done, unplug the back-up light switch electrical connector, and unscrew the switch from the transmission case. Apply RTV sealant or Teflon tape to the threads of the new switch to prevent leakage, install it in the transmission case, and tighten securely before plugging in the electrical connector. Finally, lower the vehicle and check the operation of the back-up lights.