Rear Light Cluster Leaking :(

[rubyeskimo]

Hi everyone, had my first polo (MKIV, 2005) for 5 months and just realised the right rear light cluster is leaking. To be honest, did not realise till the R indicator kept doing that slow ticking thing when one of the bulbs has gone. Got round to looking at it today and the bottom of the light cluster is full to the brim with water! Emptied it all out and it's corroded the inside of it (no idea how long it's been like that). Nothing wrong with the bulbs, but it's chucked it down this afternoon and there is water in there again already! Checked the seals and there doesn't seem to be anything wrong, but clearly there is. Any advice?