If it's the same as mine, take the inner rear indicator out and put on a radiator for at least a few hours.
Then put some sealant around the seal (I just used silicone grease) and reinstall.
I've just caught my left one early and (hopefully) fixed it this way.
My right, I left too long and had to buy a new (genuine)one.
The left had no condensation, the right, did.