Originally Posted by big_wasa

The ecu switches the earths. You will want the fast / high speed switch. Slow / low is used for the Ac.

You will want some cable capable of around 25a min. 2.5mm2. Into the relay and out to the fan on the big spades. 12v into the relay, the ecu earths it on the opposite pin. And the fifth tiny pin is a separate chassis earth.