Why would the headlights on a 1999 Pontiac Montana keep getting moisture inside them eventually causing the bulb to blow?
The glue used to join the clear and white plastic section of the
lens (front and back, respectively) is of very poor quality. The
glue comes apart with time and if you use a screwdriver you can
actually separate the lights very easily, this is why water can get
it and cause moisture.
You will have to go to a local auto shop and buy black or grey
gasket maker, it comes in a silicone like caulk tube.
With a flathead, pry the lights apart around the merging point,
if they are old they will come apart really easily. Clean the seam
path and apply new glue in the tracks, it should be about 1/8" of
glue in the channel.
You can now press the two pieces together and let them dry for
24 hours to fully cure inside the house. Make sure the bulbs are
removed so the glue can dry properly or else it will smoke when
Im a headlight retrofitter so i have dealt with all sorts of
housings, believe me this will fix the problem for many years to