* Brake Light Switch: This is the most common cause. Located near the brake pedal, it's a simple switch that closes when you depress the pedal, completing the circuit to the brake lights. It can wear out, become misaligned, or simply fail. Test it with a multimeter (checking for continuity when the brake is pressed) or replace it as a first step. This is a cheap and easy fix.
* Wiring Harness: Worn or damaged wiring between the brake light switch and the tail lights is possible, especially in an older vehicle. Look for broken wires, corrosion, or loose connections, particularly near the switch and around the taillights themselves. Check the connectors at both ends.
* Brake Light Circuit in the Wiring Harness: Even if the main wiring looks good, a break within the brake light circuit section of the harness is possible. This usually requires tracing the wiring diagram to isolate the problem.
* Ground Connections: Poor ground connections at the taillights or elsewhere in the circuit can prevent the brake lights from working while the running lights (tail lights) still function correctly. Clean and tighten all ground points related to the taillight circuit.
* Central Tail Light Assembly Switch: Some vehicles have a switch inside the tail light assembly itself. While less common, this could be defective.
* Body Control Module (BCM) (Less likely): In some vehicles, the BCM controls the lighting. However, this is less likely to be the problem if the tail lights work.
How to proceed:
1. Brake Light Switch Test/Replacement: This should be your first action. It's relatively inexpensive and easy to access.
2. Visual Inspection of Wiring: Carefully examine the wiring harness running from the brake light switch to the taillights for any damage.
3. Ground Connections: Clean and tighten ground connections.
4. Multimeter Testing: If you're comfortable with basic electrical testing, use a multimeter to check for voltage at the brake light sockets when the brake pedal is pressed. This will help pinpoint the location of the fault.
5. Wiring Diagram: Consult a wiring diagram for your specific year and model Aerostar. This will be helpful in tracing the brake light circuit if the problem isn't obvious.
If you're not comfortable working with car electrical systems, it's best to take your Aerostar to a qualified mechanic. They will have the tools and expertise to quickly diagnose and fix the issue.
