To get further help, you really need to post a wiring diagram of the brake light circuit for the car in question. Random guessing is going to get you nowhere. The path from battery to ground needs to be diagrammed to figure out where the fault is occurring, it clearly isn't the brake switch or the bulbs.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]