• You'll need to take it to some auto shop. It seems like a wire is frayed and touching.
  • Assuming the bulbs are ok check the fuse, if that is ok check the brake light switch. I'm not familiar with your Merc but the switch is usually operated by the brake pedal so will be behind the pedal in the footwell
    • Linda Joy
      The vehicles I worked on had a disconnect switch on the front of the brake pedal so that when the pedal was up it would disconnect the switch, then when the disconnect switch was broken by pressing the pedal the connection would light the brake lights. It was on the front of the brake pedal at the point where it touched the disconnect switch. But this was not on a car, but rather Aircraft support equipment years ago.
    • Professor Yaffle
      That's the sort of thing, usually a "make" switch behind the pedal that's pushed by the depressed pedal, completing the circuit & sending current to the brake lights.
  • Take a voltmeter and pull the fuse, then place the meter where the fuse was and press the pedal. If there's no juice, then it's something upstream of the fuse, like the relay/switch or the wiring from the switch to the fusebox. If there is juice, then it's either the fuse or the bulb or the wiring between the two points.
  • Have you read your owners' manual? Do you have a repair manual for the vehicle? I know they've discontinued the Chiltons' Manuals but you should be able to get a manual for your vehicle. Ask at the library first. But the answers you have gotten are good ones. My info is outdated.
  • Do the other tail lights work? Check & clean the earth connection from the light cluster to the body. Cars use the whole car body as the negative conductor so this carries the current back to the battery.

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