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.
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
Start with the light sensor on the brake pedal arm.
Adjust or replace stop lght switch
Start with: Check fuses, brake light switch (at brake pedal), check for trailer harness problems, check bulbs.
gas line and a brake light also a kick start for if it breaks down from no battery you can kick start it.
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.
I would start by cheking the brake fluid level. It settles down during the night and wen you start driving the fluid shakes enough to turn the brake fluid level sensor out. Maybe it is time to have a look in your brake pads, as they worn out your brake fluid level goes down. And normally that's the brake pads maintenance warning light. Good luck! Cheers Pedro
If the parking brake / emergency brake is on , or if the BRAKE FLUID LEVEL in your brake master cylinder in the engine compartment is LOW , and probably before you start the engine when you turn your ignition key to the RUN position just to make sure the bulb for the warning light works