The problem is a water leak around the seal where the light bulb plugs in. My Dad's Ford did this same trick, and it drove him nuts. The real problem was getting moisture on the bulb, which drastically shortened its life.

To correct it, I took the easy way out by removing the bulb holder and aiming a hair dryer into the opening to evaporate the moisture.

Once the light was dry again, I coated the rubber seal of the bulb holder with a liberal covering of electrical grease (so as not to damage the plastic.) With the grease-coated seal in place and the retaining ring screwed down securely, there hasn't been any more water getting into the headlight housing.

One other problem he had that may be applicable to your headlights is UV damage to the front lens. Out here in the desert a new headlight goes opaque in about 4 years. I use rubbing or polishing compound on a cloth to cut through the damaged layers of plastic, or if it's really bad, I use a linen buffing wheel coated with a wax-based polishing compound and chucked into a drill. Follow up with a liberal treatment of UV inhibitor like ArmorAll or 303 Protectant.