To replace a leaking heater matrix is fiddly and time consuming but a Haynes manual will explain the procedure fully. If it is leaking it need doing promptly, otherwise you could lose all of your coolant and ruin your engine. How do you know it is leaking? My daughter has just paid a garage £340 to have the matrix replaced in her 307cc but that will have been more complicated than your Corsa.
It isn't something more simple is it? For example your heater set to re-circulate? That would steam up your car because no fresh air would be coming in.
Another cause of steaming up can be a clogged pollen filter.
ChicksFan, December 2011
