Heater blower motor earth?

Good day to you all.

When i put my blower fan on full speed on man 1998 300Tdi Defender i get what i think is a earth issue because my engine weter temp gauge goes up the 3/4 mark.

I have a wiring diagram and see that there is a earth on the switch levers and i have checked that and it is fine but should there be a earth on the fan motor or does it earth through the body of the heater box?

I don't reacall a separate eath, it is bolted to the bulkhead.

The water temp sensor is a varistor and as it get hotter the resistance falls, cold water = high resistance : small deflection, hot water = low resistance : large deflection.
Your description is that when the heater fan is on (which uses a resistor on low and direct 12V for high) your varistor changes/alters its output or more voltage gets to the coil in your gauge.

Does the resistance of the sensor change --- I wouldn't think so.
Does the voltage on the dial change --- more likely.
Check the earth to the gauge/instruments first, take a wire from the guage directly to an earth other than the bulkhead.
If that works -- Earth the bulkhead to the engine/gearbox.

check EARTH headers C0287 and C0286 for corrosion or water ingress and bulkhead connection