The brake lights are on with no one pressing the brake pedal

Are you brake lights coming one when you press the brake pedal? If not, the issue is probably a dead battery. If the lights are coming on, it is probably a defective brake switch (which tells the car when you are pressing the brake).

try adjusting the switch away from the pedal as it requires to be free when the pedal is moved so that the contacts in the switch can activate and turn on the lights.

While I'm not overly familiar with Mercedes, I'd assume that it still has a brake light switch located on the bar of the brake pedal itself. This switch is typically just a "plunger" style switch, where when you push the pedal down as if applying the brakes, the plunger extends outward turning the brake lights on. When the pedal is released, as if letting off the brakes, the plunger is depressed causing the lights to shut off. If that plunger gets stuck in a depressed position, the lights will remain off at all times. Check that first, along with the brake light fuse and/or relay. Good luck.

Have you checked the brake light switch on the brake pedal ?

Did you adjust the brake light switch correctly? To adjust the brake light switch, first press the brake pedal down a few inches, then pull the switch plunger rod out of the switch toward the brake pedal arm, as far as you can without using excessive force. Next, return the brake pedal to the pedal stop position against the pedal stop and pull the brake pedal toward the rear to push the brake light switch actuating plunger rod into the switch housing limited by the travel of the brake pedal arm rear travel stop position. This procedure automatically adjusts the brake light switch correctly.
If the brake lights still don't turn off with the brake pedal released, check the brake pedal arm for sticking or binding that is preventing the brake pedal arm from returning fully to the pedal stop when the brake is released.

I'm assuming you're talking about an automatic transmission. When you press on the brake pedal, you engage a switch under the pedal which turns your brake lights on, and also engages a solenoid in the shifter which allows you to shift the car out of Park.

That said, are your brake lights working? If your brake lights come on when you press the brake pedal, chances are the shifter solenoid/actuator is broken. If your brake lights DO NOT come on when you press the brake pedal, chances are you have a faulty brake light switch.

Regardless which part is to blame, these are pretty simple and inexpensive repairs.

Good luck, and if this helped, please rate my answer!

Your brake pedal....at the top under the dash a switch attached to the pedal brake pedal switch spring is hooked it might have came of or might need adjustment counterclockwise in the adjustment.adjust and inspect

If your brake lights are lighting when you press the brake pedal then your gear shifter lock solenoid could be bad and need replacing. If your brake lights do not light up when you press the brake pedal, then your stop/brake fuse is probably blown.

Replace the brake switch above the brake pedal.

test the brake light switch which is located at the top of the arm the brake pedal is attached to, there should be 12 volts at both sides of the switch when the brake pedal is pushed.