What car do you have? Diesel exhaust will make the port side light seem dimmer as soot accumulates on the light.

Try taking the lens out and cleaning it. Water can get in there and cause mold if the gasket leaks.

Mike

PS- put your car-model-year in your signature.

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'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E