Progress!

[ONION]

wire the fan in independantly.
dont add it to a bunch of wires, or add it to an existing block.

youll find there is a wire that runs into that block, that if its corroded or loose etc makes the car run like a bag of shit.

make sure ALL the wires and wired in securely.

ADZ... finally got to say it

[nissan_turbs]

wire the fan in independantly.

It is, straight to the wire that goes to the battery, and straight to ground.

Maybe be a grounding issue, will have a look in a sec.

[ONION]

have you connected it to another power cable?
or wire the fan with a connector dirrect to the battery terminal?

if so then it must be your earth mate.
or a faulty fan.