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OK.. let'sa give it another go..

The RESISTOR behind the headlamp is the Dropping resistor for the LOW FAN.. it has nothing to do with the resistance of the gauge sensor.
The gauge sensor is the single wire sensor at the thermo housing. Next to the other one,[ meaning the one you unplugged to get the high fan on]...It has a RESISTANCE value ... if you measure that RESISTANCE [inside the sensor] and compare it to the temp/resistance chart, that will tell you if your gauge is reading correctly...forget the RESISTOR behind the headlamp.. that is a different RESISTOR [low fan]. which we know is OK b/c you have LOW fan. Forget that one for now and go find the gauge sensor and measure its resistance...
There are many, many resistors in your car...don't confuse one for another..