Most Likely Culprits:
* Bad Ground: This is the most probable cause. A poor ground connection somewhere in the electrical system prevents the lights from working and also triggers the door ajar chime (the dinging). The lack of a good ground prevents the system from properly registering that the door is closed, even when it's shut. Check the main ground cables connected to the chassis; corrosion is a common culprit. Look for loose or corroded connections at the battery negative terminal, the engine block, and the body. Clean them thoroughly with a wire brush and apply dielectric grease.
* Faulty Body Control Module (BCM): The BCM controls many electrical functions, including lighting and the door ajar warning. A malfunctioning BCM could cause both the light failure and the constant dinging. This is less likely than a ground issue, but possible.
* Blown Fuse(s): While less likely given the scope of the problem (both tail and dash lights out), check your fuse box for any blown fuses related to the lights. Make sure to check the fuses for the tail lights, the instrument panel lights, and even any related relays.
* Wiring Issues: Damaged, corroded, or disconnected wires in the wiring harness, particularly those running to the rear of the vehicle for the tail lights and those to the instrument panel for the dash lights, can cause these symptoms. This often happens near the hinges and door seals where wires are constantly flexed.
Troubleshooting Steps:
1. Check the Fuses: Start by inspecting all relevant fuses in the fuse box(es) — both under the hood and inside the car. Replace any blown fuses.
2. Inspect Ground Connections: This is the most important step. Carefully examine all major ground points. Clean and tighten any corroded or loose connections. Pay close attention to the ground points near the battery, engine block, and the body of the car.
3. Check the Wiring: Visually inspect the wiring harness, particularly around the door hinges and under the car. Look for any signs of damage, fraying, or corrosion. Pay special attention to the wires going to the tail lights and the instrument panel.
4. Test the Lights Directly (if you have a multimeter): If you're comfortable working with electricity, use a multimeter to test the power and ground at the light fixtures themselves. This will help pinpoint whether the problem is with the lights or the wiring leading to them.
5. Consider the BCM: If all other checks fail, the Body Control Module (BCM) might be faulty. This requires professional diagnosis and potentially replacement.
Important Note: If you're not comfortable working with car electrical systems, it's best to take your Pontiac Transport to a qualified mechanic. Improper electrical work can lead to further damage or even injury.
