You can test the live to the fan switch, but this then turns into three wires that go to the resistor built into the top of the fan housing. You can just see it be peering into the scuttle area.
The fan live and neutral come off the resistor, but you cannot get to the wires etc without removing the dash etc.
If you remove the fan you can test this direct to the battey so you know this works, and the connections to the fan may need a squuze with the pliers to tighten them up.
But I'll bet its that damn resistor. I fixed mine by getting a screwdriver and trying to clean the connectors on the resistor (with battery disconnected), but a month later it isnt working again!
I was thinking of rewiring the circuit and putting the resistor somewhere I can physically get to it...but its a dash out job again.