I have a 1987 740 GLE. When I start the car and turn on the headlights, the brake warning light on the dash illuminates. If I turn the headlights off, the warning light illuminates when I depress the brake pedal.

I have checked all the bulbs and they are all working fine. I see no burnt bulbs. I removed the driver/rear brake light assembly from the car because the lens was cracked and dirt was inside. I cleaned out the inside, inspected the contacts for all of the bulb housings then used silicon to seal up any gaps and cranks on the reflectors. I checked the grounds as well.

While inspecting my car, a mechanic told me to check the grounds because the brake light was dimmer with the headlights on? He thought a bad ground might be the culprit, but the grounds seem fine.

Any advice on what might trigger this dash warning light, if all the lights seem to work?
Thanks!