Moisture in indicator is almost unavoidable unfortunately, very fine cracks will cause moisture to get in there when it's humid/raining...either go nuts with some silastic or spend hundreds on new tail light assemblies
If there's moisture in the boot, you might have the same problem as me. Remove your spare tyre - in the well that it sits in, there will be a couple of holes in the bottom. I think there will be at least 1 small hole (10c piece size) and one fking huge one (like 10cm diametre) There should in theory be a rubber plug for at least this large hole....if you've got the same issue as me, then you won't have a plug lol. Water can spray up off the road and into this hole, in turn finding it's way into the boot. Just cover the hole with something, like a flat piece of rubber that you can cut to size (a little larger of course) and sit over top, or a flat piece of wood....or whatever you want lol