Hi dash clocks radio and steering wheel off, don`t think you have air bag so no dangers, unplug dash switches and remove heater control fascia panel, some Phillips under there, to undo, . remember your digital clock wires, don`t wreck the connection, pop clock out and remove wiring plug. heater matrix unit is simply a pair of 13mm nuts, undo them squarely , don`t agitate them too much as they are pressed into the scuttle, and if they lose their fixing they are 1st class shits to undo. , 2 pipes need undoing in engine bay above steering rack, you could of course just bypass the heater, with some 18-20mm tube, drain your coolant antifreeze mix into a can before you start
there may be a drain plug on a lower corner of the rad usually round plug the size of a centine / (5 p here), back to the heater when dash is out and 13mils are undone and hoses onto heater box are removed the unit will wiggle out of its location quite easy,
and the matrix slides in from the side 2 Phillips, you probably need a replacement matrix, as a cheap repair will leave you to fix again
personally if I was 81 I`d bypass the matrix with the tube between the rubber pipes and get a youngster on the case, either way have a nice weekend