Appreciate this in an old thread but I thought I would reply as I’m having the same issue now.
New to the van world in Sep with a T6 kombi purchased with aftermarket front bumper/spoiler. Had condensation appear on the inside of the driver side fog after I swapped out the halogens for LEDs. Dried it out a number of times (with a heat gun) and it came back. Took it to the dealer, they replaced the whole fog unit it still came back. Dried it out again and stuck the halogens back in and guess what - back again! At a closer look, it could be the bumper directing rain (which we’re having lots of at the moment) directly onto the top of the fog light. With the intermittent cornering (fog light switching on), it’s not getting a chance to dry out properly. Other than change the bumper, I’m not sure what else to do here.
Has anyone had any more luck with this issue? Any more thoughts would be appreciated. Cheers.