1986
Start with replacing the bulb.
its a rear bulb failure indicator if it comes on when brake is first depressed it is a brake light bulb if it is on when you start the car it is a ride light or park light
The "3rd" brake light has been out since at least 1994.
To reset the brake pad light on a BMW X3, start by turning on the ignition without starting the engine. Then, press the gas pedal three times while holding the brake pedal. After that, turn off the ignition and then start the engine; the brake pad light should be reset. If the light persists, it may indicate an issue that requires further inspection.
try your brake light swtich on the brake pedal and check wires
check the play in your rear brake lever, may be too tight and activating the brake light switch. or the front brake lever may be depressing the switch in the control housing. or you have a bad switch. or, when you rewired it, after installing that cool skull tail light, you got the running light wire and the brake light wire backwards. D
To reset the brake light on a BMW 645Ci, first, turn the ignition to the "On" position without starting the engine. Press the accelerator pedal three times in quick succession, then turn the ignition off. Start the car again, and the brake light should be reset. If the light remains on, check the brake fluid level and the brake light bulbs for any issues.
Start with the light sensor on the brake pedal arm.
Start with: Check fuses, brake light switch (at brake pedal), check for trailer harness problems, check bulbs.
Adjust or replace stop lght switch
The brake light switch in the hand brake is probably broken or the tab that pushes it is bent. You can also see if your throttle controle is pushed all the way up to the master cyclider.
gas line and a brake light also a kick start for if it breaks down from no battery you can kick start it.