If you look on the outside of the car where the wipers are there is a plastic cover on the drivers side. If you lift the bonnet up for more access you will see (im going from memory here) one plastic screw that is lifted up with a flat heat screwdriver, then I think theres a long plastic clip which you lift up to remove the whole cover, there you will see where the heater box is.
If you do a search there will be some more information, I think you might just need a working second hand unit.