Mark 1 or mark 2? Not that hard really. Mark 1 then take out fuse box (pulls backward and drops down a bit) get a long screw driver and you should be able to unscrew it with a bit of patience.
Mark 2 you can take out the heater vent, pull the vent director out and there is a screw in there at the bottom, undo that and the full thinkg will pull out, push the heater pipes out the way and the speaker twists out of the dash board. It wont be easy to put an after market speaker into a mark 2 dash.
Wanna buy some nova GTE parts by any chance? lol