I'm not sure, but the fault may be as simple as a bad brake switch ($10 part). This part is located above the brake pedal in the interior and is known to go bad. When it goes bad, it believe it trips the ETS light, the third brake light doesn't work, and the cruise control stops working.

I think it's worth a shot replacing this switch before shelling out $1200 to replace the ETS pump.

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95 E320 Cabriolet, 169K