replaced the heater matrix in my old Citroen ZX, had to take the whole dashboard off including steering wheel, centre consoles, hot air ducts and motor, it was scary sitting in the car just looking at the firewall with the loom just draped over it If you do replace it, try getting one from a scrapper. You get to make all the mistakes breaking clips and trim on the donor car and removing bits that you didn't need to.
Did the same thing replacing it on a 306 D Turbo a few years ago, never again to be honest.
As for the orginal post, there should be a valve you can turn to turn off the flow to the heater matrix, as the pipe feeding it forks off into the matrix before coming back around into the engine. It should be right at the back of the engine bay, infront of the bulk head. However it does mean it will be rather cold in the winter! But it will get it fixed for now.
Good luck either way!