Err dont want to say it but might be windscreen? My old Mk2 had a new windscreen and it was never the same,esp with water peeing in through onto the fuse box!!!
When i went back to said windscreen fitters(Auto Glass) they said "Ohh we noticed rust on the seem but didn't think anything of it as its common on these older golfs, we could try to seal it for you now"

I said something to do with them being inbred and how would you like a bucket of rain water in your car, and went home and found some clear silicone and poked it under my windscreen rubber and sealed it myself,yes its a messy way of doing it but I was desperate!!
Anyway its probably best to get your head under the dashboard with a small torch and have a good look as if you catch it early enough you'll less likely ruin your heater etc and prevent that lovely rotten sound deadning smell...