Timm's BMW Condensation In The Front Light Clusters
I have seen many E38's with condensation inside the front light clusters, this is especially noticeable in cold weather with rivulets of water seen inside. This really spoils the appearance of the front of the car and can diffuse the headlight beams. However, there is an easy fix for this problem
What not to do!
The initial impression is that water is entering the headlight enclosure past the large seal between the enclosure and the front glass (or plastic on the facelift models). The usual fix tried is to use a silicone sealant on the gasket but this usually makes the problem a lot worse. This is because most silicone sealants have a solvent base that evaporates during curing and this produces moisture within the enclosure. More serious is that many sealants give off acetic acid as part of the curing process, this can damage certain metals and may discolour the reflectors.
I have also seen attempts to remove the moisture by bunging in packets of desiccant. This seems a good idea except that as soon as the enclosure heats up the desiccant will release the moisture straight back to the coldest part of the enclosure which is the front glass.
The usual cause is not water getting past the front seal but is due to moisture being present in the enclosure when it was last closed. This can occur when changing a bulb in very damp weather, any moisture within the housing will accumulate on the front glass as this is the coldest part. It is certainly worth checking the front seal but this is rarely the problem, if the seal has been disturbed in the past then simply coat the seal with silicone grease, this is quite sufficient to stop water getting past the seal as long as it has not been damaged.
The method of getting rid of any moisture in the enclosure is simple, wait for a dry day and take off one of the access covers on each enclosure, start the engine and switch on the headlights (full beam). The heat generated will drive the moisture from the enclosure within 30 minutes. Switch the engine off and switch the lights off and then immediately replace the access covers. This will stop the return of any moisture, job done!