1999 Dodge Avenger ES,I notice water condensation in my fog light. Any recommendation on how to fix it, or do Ijust replace it.

Drill a 1/4 inch hole in the bottom of the fog light housing. This will allow water to drain and air in to dry up any moisture.

I was thinking more in the line ofhow to find the leak and sealing it, as putting another hole into the light may justperpetuate the problem.

Good luck with finding the hole. The water is just condesation that is built up over time. Its just best to either to replace it or drill a hole like aven mentioned.

Throw in oven at lowest setting removing bulb , this will dry out the moisture ...
Take a heatgun and heat up the perimiter and gently pry it apart , put a bead of clear silicone RTV sealant in the crack reassemble wipe extra smooth allow to cure