I was talking to some friends, and the reason floor pans on Mk2 Golfs go, and mainly on the passenger side is the heater fan inside the car. You have a vent opening by the windscreen which leads to the fan.
If this leaks, your carpets get soaked, and over time it takes the floorpan.
It has to be severe though for it to happen quickly.
Easiest way to spot it is to find out if your fan is noisier that it should be. This would indicate the spindle on the top of the fan leading to its armiture is corroded, which means its got water eating at it.
I'm not a mechanic sso I dont know how true this is. My fan had defintely got corrorded, but the floorpan was okay, and the Oak Green Mk2 we had was exactly the same.
However, my Dreg Driver (spares mule) leaked like a sieve and had a rotten floorpan and it had been left to the elements for far too long.
Tin