1. The Obvious Checks:
* Bulbs: This is the most common cause. Check all three brake light bulbs (center high-mounted stop lamp (CHMSL) and two tail lights). Replace any burned-out bulbs. Make sure they're the correct wattage.
* Fuses: Consult your owner's manual to locate the fuse(s) for the brake lights. Check them with a test light or multimeter to ensure they haven't blown. Replace any blown fuses. Note that there might be separate fuses for the CHMSL and the tail lights.
* Brake Light Switch: This switch is located near the brake pedal. It activates when you press the pedal. A faulty switch is a common culprit. You can test it with a multimeter (check for continuity when the brake pedal is pressed). Replacing it is relatively inexpensive.
2. More Involved Checks (requiring basic electrical knowledge and tools):
* Wiring: Check the wiring harness leading from the brake light switch to the tail lights and CHMSL for any breaks, corrosion, or loose connections. A visual inspection is a good start. You might need a multimeter to check for voltage and continuity at different points in the circuit.
* Brake Light Switch Connector: Inspect the connector itself. Corrosion or loose connections at this point are common issues. Clean the contacts with electrical contact cleaner if needed.
* Ground Connections: Poor ground connections can prevent the lights from working. Check the ground wires at the tail lights and CHMSL for corrosion or loose connections. Clean or tighten them as needed.
* Body Control Module (BCM): In some cases, a faulty BCM can cause brake light issues. This is less common but a possibility. Diagnosing BCM problems usually requires a scan tool.
* Wiring harness to the CHMSL: The wiring to the high-mounted brake light can also be a problem. Look for damage or corrosion.
3. Getting Help:
* Auto Parts Store: Most auto parts stores can test your brake light switch and bulbs for free. They can also help identify the correct parts if you need replacements.
* Mechanic: If you're not comfortable performing electrical repairs, take your Mustang to a qualified mechanic for diagnosis and repair.
Important Safety Note: Driving with non-functional brake lights is dangerous. Do not drive your vehicle until the problem is resolved. If you suspect a more serious electrical problem, it's best to have it towed to a repair shop.
