I believe the front hole is for headlamp washer pump if car was fitted with the option.

happened twice on our '97 C280.

The original plug last 7 years for us. In fact It outlast the plastic tank which cracked 1 year prior. I transferred the plug over and did not see anything abnormal.

First replacement (1dollar plug) I bought from dealer was plastic instead of rubber- lasted 1 year before pinhole seepage occurred.

2nd time MB changed back to rubber & I hope this will last at least few years.

Winter washer fluid up here in Toronto seems to `promote' the rubber degradation according to the part guys at 2 dealers.