There are several symptons:

Car is steaming up when heating is on
Water is coming into the car through the vents (I think) when it is being washed
Car won't start when it has been running (as though it is damp) - fixed by a blast of WD-40

I have a feeling there is a blockage in draiange somewhere, but I'm not sure... help please!

Andrew Ching, February 2006

Most probably it is the heater that is used to heat up the cabbin of the punto. The common fault is that the heater brakes and the water is leaking inside causing the stam up effect. The heater itself is not too expencive (around 50EUR) but to change it is quite a job because you need to disconnect the cooling fluid hoses from underneeth the bonnet and then dismantle the intire cabbin. Everything must come off in order to replace the heater. I do not recomend unexpirienced people to that. I took my punto to a mechanic and it took him 3 days to replace the heater. This is a problem that must be mended because it is drainig cooling fluid and could cause your car to overheat

Dex, November 2007

I have a P reg Fiat Punto and although this isn't an answer ... if it helps I have the same problem. I have tried having the windows open, the heating on and off, different conbinations. Its actually getting to the point I can hardly see where I am going. Its winding me up beyond belief and thinking of getting ird of the car which is a shame cos I really like it other than this problem.

Peter Bailey, February 2006