Hi James.
How many wires are going to the motor, you maybe trying to fit a gold resistor to a four wire 3speed fan?
We need more info, personally I would look for a two wire 2speed fan with silver or gold resistor and swap out the fan you have on now.
Such as this below, but change the gold resistor as it as not been fitted great, nothing to do with me by the way just had a quick search.
Double check its for a diesel
https://www.ebay.co.uk/itm/rover-75-...EAAOSws4NehQAt