Check to see if your ground wire is still conected to the body.
STICKING BRAKE LIGHT SWITCH AT THE TOP OF THE BRAKE BEDAL
The brake light switch located above the brake pedal is broken or out of adjustment.
Check the brake light switch on the brake pedal. Might have to replace it.
Just had the same question and found the answer... According to my manual for a Mercedes-Benz 560SL 1986 It is the Exterior light failure indicator lamp. "The light failure indicator comes on with the key in steering lock posion2 and will go out when the engine starts. If it comes on with the engine running, it indicates a a light failure. If an exterior light fails, the indicator will come on only when that light is switched on. If a brake or turn signal lights fails, the light failure indicator will come on when applying the brake or actuating the turn signal and stay on until the engine is turned off. CAUTION! If additional lighting equiment is installed (auxiliary headlamps ect.) to avoid damage to the failure indicator system, be certain to connect into the fuse before the failure indicator monitor unit.
1986
Low brake fluid level or Parking brake not released completely
possibly a bad brake switch located on the brake pedal assy
Bad switch on the emergency brake handle.
instert key into slot right below oil light indicator.
Check the fuse, check the bulbs and check the brake light switch.
Brake light switch is located on the brake pedal arm. Unbolt & bolt in new one. Wiring will unplug & plug back in to new switch.
check your e brake