1. Check the Bulbs First (Easiest Fix):
* Access the bulbs: This usually involves removing the inner trim panel of the taillights. Refer to your owner's manual for specific instructions.
* Inspect the bulbs: Look for any signs of damage, breakage, or burning. Even a slightly loose bulb can cause this issue. Replace any faulty bulbs with the correct wattage.
2. Check the Brake Light Switch (Most Likely Culprit):
The brake light switch is a small switch located near the brake pedal. When you press the brake pedal, it activates the switch, completing the circuit to illuminate the brake lights. If it's malfunctioning, it might be stuck "on."
* Locate the brake light switch: Consult your owner's manual for the exact location. It's usually near the brake pedal, sometimes accessible from under the dash.
* Inspect the switch: Look for any visible damage or signs of wear. Sometimes, they just get stuck. Try gently pushing on the switch while testing the brake lights.
* Test the switch (if possible): Some switches can be tested with a multimeter to check continuity. If you're comfortable with basic electrical testing, this is a helpful step. If you don't have a multimeter or aren't comfortable with electrical work, skip to the next step.
* Replace the switch: If inspection or testing suggests the switch is faulty, replace it. This is a relatively inexpensive and easy repair, but you'll need the correct replacement part for your model.
3. Check the Wiring Harness (Less Likely, More Difficult):
A short circuit or damaged wire in the wiring harness leading to the taillights can also cause this problem. This is a more involved process.
* Visually inspect the wiring: Carefully examine the wiring harness from the brake light switch to the taillights for any signs of damage, fraying, or corrosion.
* Use a multimeter (if comfortable): Test the voltage at different points in the harness to pinpoint the location of a short circuit.
* Repair or replace damaged wiring: Repairing or replacing sections of the harness requires some electrical knowledge and soldering skills. If you're not comfortable doing this, take it to a mechanic.
4. Check the Fuse (Unlikely, but Possible):
While less likely to cause constantly illuminated lights, a blown fuse in the wrong place *could* cause strange electrical issues. Check your owner's manual for the location of the fuse box and diagram to find the correct fuse for the taillights.
5. Consult a Mechanic:
If you've checked all the above and the problem persists, it's best to take your Ford Focus to a qualified mechanic. There might be a more complex electrical issue, a problem with the BCM (Body Control Module), or another underlying fault that requires professional diagnosis.
Important Safety Note: Always disconnect the negative terminal of your car battery before working on any electrical components. This prevents accidental shorts and shocks. If you're unsure about any step, consult a professional mechanic.
