You have a "dead short" somewhere in the circuit. It would appear that a wire has rubbed bare and is touching metal somewhere between the brake pedal and the bulb. The first place I would begin checking is around the rear bumper. Look for places where the wires leave the wire shield and head up towards the lights. Especially where they pass through metal. Perhaps a grommet is missing allowing the wire to rub on metal. There's alot of vibration back there. Have patience and determination and you'll find it. Good Luck.
Is it the parking brake light or the ABS light? is it red light or amber?
The brake light in the Audi A4 display means there is an issue with the brake system. The most common cause of this warning light is low brake fluid.
Check the brake fluid level in the reservoir there is a sensor there that will make the light come on if your level is low.
This is a sign that you need to have the brakes inspected, soon. Worn brakes causes the brake fluid reservoir level to drop causing the brake and abs lights to come on.
Once you correct the problem the light will go off. Can be low of brake fluid, parking brake might be partially on, defective Master Cylinder, etc. Do not ignore this light as this can cause your brakes to fail.
If you are losing fluid and the pedal is low, you have a brake fluid leak.
A problem has been detected with the ( Antilock Brake System ) It could be a fuse , a sensor , or some other problem As long as the red " BRAKE " light in your dash is not on , you still have " normal " brakes
The first thing to check is low brake fluid. A leak in the system or worn brakes causing the fluid to be too low is the most common cause for the light to be on.
Your brakes are failing
Have you checked to make sure your brakes do not need replacing? Also, check your brake fluid. Low fluid can cause that light to appear as well.
check fluid level
Could be low on brake fluid. If it is low you either needs brakes or you have a leak in the system.