A couple of thoughts.

The problem may be a bad brake switch, located above the gas pedal. This may be a cheap fix (~$15). Since it is a systematically weak part across many Mercedes models, you won't be wasting your money.

If the brake switch isn't the problem, then it's beginning to look more like a bad ETA (electronic throttle actuator). I agree with J. M. van Swaay and you'll need to fix this part. I rebuilt mine - the wiring insulation turned to dust on my workbench.

You'd have three options available to you:
1. replace the ETA with a new unit (~$1400)
2. have the unit rebuilt (~$400, from Beckmann Technologies?)
3. replace the wiring yourself

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95 E320 Cabriolet, 169K