Evening,
Common on mk2's.
The heaters motors are sh!te, take the motor off from below by pushing the little tab back and turning the whole motor anti-clockwise.
You Will have to take the lower trim panel off first!
Its only held on with about 6 screws.
Try spraying some wd40 on the motor spindle to free in up, plug it back in, and put it on speed 3 (or 4 if its a 90 spec) and let the bugger run, just to get it freed up.
The heater matrix is built into the heater unit and is basically a miniture radiator, They do get bunged up quite easily, and even though you have to take the dash out to replace, it is not a hard job.
Oh and by the way, if your heater motor only works on full speed, its the resistor pack attached to the motor, not the motor itself. they are about ?30 new, or just rob about 5 from a scrap yard. You will need 5 because at least 4 of them will be screwed.