[ACHiPo]

Got a little bit of data today. At this point I'm pretty sure the problem is not on the heater portion of the circuit.

I pulled the heater switch out of the dash and checked continuity. The light green wire is shorted to ground (not too surprising since it is connected to the W/G wire which is also shorted to ground when measured at the fuse block).

The orange wire connecting the heater switch to the heater fan is 49 Ohms, so seeing the motor windings before ground. This seems ok.

The heater switch is ok.

Because it was easy, I pulled the connector off the wiper motor just to make sure it wasn't shorted and it's not. The light green wire is still a short to ground.

It seems like the short has to be between the wiper switch and ignition light, ie I'm back to suspecting the short is at the tach or fuel gauge. Will drop the dash again tomorrow and pull the tach and fuel gauges (again). If nothing else it gives me a chance to reinstall the Smiths voltage stabilizer on the fuel gauge.