Well I am not real sure how you could find the leak, but I guess you could remove it from the car, remove the light, then fill the assy up with water. Make sure the outside is dry. Seal it off and see if the water slowly starts to trickle out. Just a thought, not that anyone has done that. Just made me think of when someone posted about washing the inside of there headlight assy.
Come to think of it, they mentioned how to get the condensation out too! On a warm day, remove the headlight bulb and let the assy air out. The warm air will "dry" in the interior of the assy. Once it is dry, replace the bulb. If you have a good seal, with no air leaks, problem solved.
Hope some of this late night rabbling helps you out some.
