Lovely car James.
The heater fan is fed by only 2 wires, red and black, and they should be visible under
the passenger side, along with a connector block. I'm no electrician, but current draw
could be measured at this point. The fan speeds are 'regulated' by the resistor mounted
on the top left side of the scuttle. This is maybe another point that current draw could be measured.
Someone good with sparks will help i'm sure.
The entire heater box has to be removed to gain access to the fan motor. It can be done without removing
the dash. You'd need to remove the centre console. It hangs in place from the 4 x 6mm nuts in the scuttle, but
could well be 'glued' to the firewall by the factory insulation.
Hope this helps, Nigel