Headlights fog as a result of moisture being present inside the headlight chamber. The fogging occurs when water in moist air condenses on the interior surface. Moisture enters the headlight chamber through cracks, bad seals or gaps in the housing. Once the moisture is inside the chamber, heat causes the moisture to evaporate, but because the moisture rich air cannot easily escape the confines of the headlight, the moisture then condenses on the interior surfaces.
Disconnect the headlight housing from the vehicle. For headlights connected with bolts or screws, use a screwdriver or socket wrench to remove the bolts or screws to free the headlight assembly. For vehicles with retaining clips, find the clip joint and disconnect the headlight assembly to free the housing from the connection points. Disconnect the headlight bulb's electrical connection to allow full access to the housing.
Thoroughly inspect the whole headlight housing for gaps, cracks or bad seals through which moisture could have entered the headlight chamber. If possible, remove the bulb and inspect both the bulb seal and the bulb entry point for cracks or damage. Note the locations of the damage for later repair.
Blow warm air from a hair dryer into the headlight housing to remove the built-up moisture. If the interior is accessible, rinse with soapy water and dry to remove any spotting. If the headlight is a sealed system, and interior access is not possible, heat the outside lens of the headlight to evaporate the inner moisture. Blow the hair dryer's flow onto the crack or damaged location to create circulation and remove moisture.
Prepare the plastic epoxy and apply to the cracks or seals requiring repair (for gasket material, clear RTV silicone may be used). Apply the plastic epoxy to the entire length of the damaged area, and smooth as much as possible to allow for better light transmission. Damaged bulb seals should be replaced.
Reattach the headlight electrical connection and replace the headlight assembly into the car chassis. Secure with the bolts, screws or clips.
If your headlight is a sealed glass system (the bulb is an integrated part) and has cracks or damage, replacement is recommended to avoid safety hazards.