The bulb indicator warning light system tests all of the rear lights for correct impedance (resistance) in the bulbs and wiring. The problem may be a license light bulb (often overlooked) or a tail lightbulb. If not one of the bulbs, there is a wiring fault such as a bad ground or corroded bulb socket.
Jim O'Neill - Chino Autotech Inc., Auto Club SoCal AAR
SAE Int'l SoCal Section Board of Governors,
Vice Chair, Service Technology Programs
NASTF Service Information/Communications Committees
Auto Value/Bumper-To-Bumper CSC Nat'l Council
ASE CMAT-L1, CA. Smog Check Technician
AMI AAM Accredited Automotive Manager