Wet Boot

[zdebx]

It will be the tail light dude, especially if the patch is just underneath the cluster.

But why only on one side? That's what buggers me 

[Khare]

So many reasons. Bolts could be too tight and have damaged the seal, or they could be too loose, or the seal could have come faulty/weak from factory, there could be a part of the metal frame around the light that has bent over the years or the light cluster itself could be faulty from factory and allowing water in.

[zdebx]

Alright, makes sense now

Replaced the lights back to original GTI smoked ones, so will be watching whether it makes any difference.

[zdebx]

Niiiiiiiiiiiice, problem solved!!! 

Today it was raining the whole day, so I thought I would check and see if the boot is wet, but it was nice and dry.

So yeah, the issue was in aftermarket lights....F*ck them - love the smoked GTI ones