Driver side rear brake lights going out too frequently

water is seeping in though the lens.you could try spray the whole assemble with clear lacquer paint that might help seal it

Since water is seeping into the light assembly, you could make a new gasket to seal out water. Use silicone sealant where the original gasket may have become stiff. Also, put some sealant around each bolt / washer that attaches the assembly to the car. If you allow half an hour or longer for the sealant to set, it will be less likely to squeeze out. As for a spray-on paint, I would not recommend it unless you can remove 100% of the car wax that may have been used previously. If you could find some NeverWet, that would work. It was supposed to be available early this year, but I haven't found any on the market yet.