* Body Control Module (BCM): This is the most likely culprit. The BCM is the "brain" that controls many electrical functions in your truck, including door locks, interior lights, and more. A faulty BCM can cause various electrical problems, often manifesting as multiple seemingly unrelated issues failing at once. This often requires replacement, and it's not an inexpensive fix.
* Fused Power Supply: While less likely to affect both systems simultaneously *unless* a particular fuse supplies power to both circuits, check the fuses related to the door locks, interior lights, and the BCM itself. Consult your owner's manual for the fuse box diagram. A blown fuse needs replacing with one of the correct amperage.
* Wiring Issues: Damaged or corroded wiring, especially around the door jambs (where wires flex constantly) or in the main harness, could interrupt the power supply to either or both systems. Inspect the wiring harnesses for breaks, chafing, or corrosion. This requires some technical knowledge and careful tracing.
* Door Lock Actuator Switch: While unlikely to cause both interior lights and locks to fail entirely, a faulty switch in one of the doors could interfere with the auto-locking function. Try testing the door lock switches individually.
* Ignition Switch: Though less common, a failing ignition switch can sometimes affect other electrical systems in the vehicle.
* Grounding Problems: A poor ground connection somewhere in the electrical system can cause erratic behavior in multiple components. Check for corrosion or loose connections at grounding points near the battery and body of the truck.
Troubleshooting Steps:
1. Check the Fuses: This is the easiest and first thing to do.
2. Test the Door Lock Switches: See if any are faulty.
3. Inspect Wiring: Look for obvious damage to wiring harnesses, especially near the door jambs.
4. Battery Voltage: Ensure your battery is providing adequate voltage. A weak battery can cause intermittent problems.
5. Consult a Repair Manual: A repair manual specific to your 1999 Dodge Ram will have wiring diagrams and troubleshooting information.
6. Professional Diagnosis: If you're uncomfortable working with car electronics, take your truck to a qualified mechanic or auto electrician for diagnosis and repair. A scan tool may be needed to access BCM codes.
The simultaneous failure suggests a more significant problem than a single blown fuse. The BCM is the most probable suspect, but thorough investigation of the fuses and wiring is crucial before reaching that conclusion. Don't attempt major electrical repairs unless you have the necessary skills and tools.
