dam it, if it was a corsa D i know the common fault with the heater as iv fixed this week.
could be the exact same thing tho.
heater resistor pack, they f**k up either down to water shorting them out or general life expectancy. common to either stop blowing or the heat isnt working.
you'll have to find the heater blower and have a look at the condition of the resistor pack. aswell as looking at the matrix for any obvious signs