* Short Circuit: This is the most probable cause. A short in the wiring harness between the brake light switch and the reverse light circuit is causing a crossover. This could be due to damaged insulation, frayed wires, or a wire touching a ground where it shouldn't.
* Faulty Brake Light Switch: While less likely to directly cause the reverse lights to illuminate, a malfunctioning brake light switch can sometimes send erratic signals that might interfere with other circuits.
* Faulty Neutral Safety Switch (NSS): The NSS prevents the vehicle from starting unless it's in park or neutral. A faulty NSS *could* send incorrect signals, but this is less likely to directly cause the reverse lights to turn on with the brakes. However, if the NSS is malfunctioning it could contribute to other electrical issues.
* Wiring Harness Damage: Damage to the main wiring harness near the transmission or rear of the vehicle could be causing a connection between the brake light and reverse light circuits. This is a more serious problem.
Troubleshooting Steps:
1. Visual Inspection: Carefully examine the wiring harness, particularly around the brake light switch, transmission area, and near the rear lights. Look for any frayed wires, melted insulation, or signs of damage.
2. Brake Light Switch Check: Test the brake light switch itself. You might need a multimeter to check for continuity and proper voltage. There are many online tutorials showing how to test a brake light switch.
3. Reverse Light Switch Check: (If your Blazer has a separate reverse light switch) Check this switch as well for proper operation.
4. Professional Diagnosis: If you're not comfortable working with car wiring, take your Blazer to a qualified mechanic or auto electrician. They have the tools and expertise to diagnose the short circuit accurately and repair the wiring safely.
Important Note: Driving with this problem is unsafe. The faulty wiring could lead to other electrical issues or even a fire. Address this issue as soon as possible.
