it may be leaking in from the top of the boot as it runs diwn the sides and there is gunk somewhere. i had a similar issue and one if my bulbs kept blowing. i cleaned all around the boot door and unscrewed the lights and cleaned out all the muck inside the light itseld, the metal and the plastc bulb holder, wiped the rubber seal and cleaned that and then wiped the bulb holder. if the clips are loose then it can get in that way.
also there should be no space between the rubber seal on the light and the frame and the light itself.
then i made sure the boit carpet panel that covers the light is on properly.
this has solved my problem for quite some time as i used to get blown bulbs everytime the weather changed.
hope this helps somewhat. a lot of muck can collect in there. i was surprised how much can get in there.
germans were never good at keeping the elements out from important stuff in the Hitlermobile
anyways if there is a gap you will have to eliminate it. maybe good old duct tape will help
