Yep, I thought mine was the rear lights (which it usually is) but when I removed the spoiler it turned out to be one of the holes had corroded and was allowing water in... I sealed the spoiler and this improved things, I still have a slight back light issue though.
Mine too, I took out all the carpets and pulled the plug out of the floor, Im fairly sure I have cured mine now, The light issue is with damp inside the light unit itself which requires me to remove the lens and reseal it...
if you remove the spare tire, there are several black rubber plugs you
can remove to allow the water to pass.
My Rt tail light seal was bad, allowing water to go into the light and in the boot. I resealed it using Black Sealer by Holts. Stuff is inexpensive so use plenty of it to do it right, the 1st time. You need to remove the tail light which was a pain in the ****, but no probs now.