I'm "hoping" to solve the problem without removing the housings from the car, but will remove them if necessary. Even covering the housings with plastic, moisture intruded and condensed on the inside of the lens when the car was exposed to condensing humidity overnight.

...anybody have an exploded parts diagram for the Euro headlight housings for a W201? It appears in addition to the glass lens bond to the plastic frame, there is a gasket between it and the main housing and the gasket for the rear access panel.

Do you have a brand name/part number for a "butyl silicone" sealer.

Duke