[old_master]

Remove the backup light socket & bulb from the taillight housing. Turn the ignition to the run position, engine off, & shift to reverse. Using a voltmeter, put a probe on each contact in the socket, it should show battery voltage, if it does, the bulb is faulty. If not, check resistance from the negative contact to chassis ground, must be less than 5 ohms. If not, look for a bad ground from socket to body. If the right b/u lamp works, it means the left b/u lamp will most likely have battery voltage to it because the left lamp feeds the right lamp. Post your results.