Troubleshooting Hints - Interior Lights

Perform before beginning System Diagnosis.
^ Check for a broken (or partially broken) wire inside of the insulation which could cause system malfunction but prove "GOOD' in a continuity/voltage check with a system disconnected. These circuits may be intermittent or resistive when loaded, and if possible, should be checked by monitoring for a voltage drop with the system operational (under load).
^ Check for proper installation of aftermarket electronic equipment which may affect the integrity of other systems (Refer to "Diagnostic Aids/General Troubleshooting Procedures,"). Diagnostic Aids
^ Refer to System Diagnosis. System Diagnosis - Interior Lights