* Faulty Reverse Light Switch: The reverse light switch is a common suspect. This switch not only activates the reverse lights, but also might be wired improperly, causing a short or unintended connection to the blower motor circuit *only* when in reverse. A faulty switch could be sending power to the blower motor circuit when it shouldn't.
* Wiring Issue: A short circuit or damaged wiring harness is a strong contender. A wire might be grounding out to the reverse light circuit or another component that's only active in reverse. This could be a frayed wire, a corroded connection, or a wire improperly routed that's touching something it shouldn't. Look carefully at the wiring harness near the transmission and the blower motor itself for any signs of damage.
* Faulty Blower Motor Resistor (less likely): While less likely, a completely failed blower motor resistor *could* exhibit strange behavior like this. However, typically a failed resistor results in only certain blower speeds working or none at all. The fact it ONLY works in reverse points more strongly to a wiring or switch problem.
* Ground Issue: A poor ground connection could be causing the problem. This is more of a general electrical problem that might manifest as an intermittent or unusual behavior like this.
Troubleshooting Steps:
1. Check the Reverse Lights: Do the reverse lights work correctly? If not, the reverse light switch is a prime suspect.
2. Inspect the Wiring: Carefully inspect the wiring harness leading to the blower motor and the reverse light switch. Look for any signs of damage, corrosion, or wires that might be touching each other. Trace the wiring carefully.
3. Test the Reverse Light Switch: If possible, test the switch itself to see if it's delivering power to other circuits when the car is in reverse. This might require a multimeter.
4. Check Grounds: Check the ground connections for both the blower motor and the reverse light switch. Clean any corrosion.
5. Consult a Wiring Diagram: A wiring diagram for your specific 1991 Lumina is crucial. This will help you trace the circuits and identify potential problem areas. You can often find these diagrams online (e.g., Haynes repair manuals, online forums).
This is a tricky electrical problem that likely requires some diagnostic skills and tools. If you're not comfortable working with car electrical systems, it's best to take your Lumina to a qualified mechanic. Improperly diagnosing and fixing electrical problems can lead to further damage or even fire hazards.
