The silica gel packs are very commonly used in the retrofitting community (people who basically make their own projector headlights for cars that didn't have them using oem parts from other cars) as a way to combat moisture in the headlights. You'd probably want to add a little sealant around the edge of the light, and correct the venting situation on the back, but a silica pack taped inside the light would do wonders to keeping the humidity down. Wouldn't be of much help if you actually had water getting into the light some how, but if it's fogging up just from built up humidity it should do its job. I imagine the heat from the headlight bulbs would help dry 'em out when the lights are in use too.