Leyland daf heater blows fuse?

Hi I have purchased a Leyland daf that has been converted to a horse box, when I try to use the heater I can put the fan lever to 1 or 2 but nothing seems to blow or get hot. Once I move fan lever to 3 the fuse blows, which is shared for the windscreen wipers so really annoying. I presume I need to get into the switch/wiring behind the controls but not even sure how to remove the panel to get to it, do you have any ideas?