Only ever worked on one air con sierra, under the scuttle panel you will see the heater motor as the forward most part, of the casing that surround the evaporator, where it goes through the bulkhead.
I can't remember whether the fan is accessible or not, just lift the panel in the centre covering it.
I'm inclined to think it is a whole unit out job, but there may well be enough slack to move it forward enough to enable the fan to be changed in situ without disconencting the air con pipes.
Prior to the above though I would check the wires at the fan are actually getting a good earth and the switched varying voltage lives, as it could just be the earth wire broken or the switch duff, so could always check power here first.
tabetha