On an '87 I would pull the power from the Starter Solenoid, battery side post, correct size "eye" terminator on the wire end.
Size of the wire would depend on the fans motor amp draw, should be on the motor, or look it up.
For 20amps and lower use 12 gauge, above 20amps used 10 gauge, same size for Ground wire
Also an inline fuse rated for the amps the motor will draw.
You can buy off the shelf electric fan relays that come with a thermostat that you insert in the radiator "fins", these are adjustable for the temp they come on and go off at.
$30-$60, most have an A/C trigger as well, so fan comes on when A/C is on.
If you just want an on/off switch you still need a relay, running TWO 20-30 amp wires through the firewall to a 20-30 amp switch on the dash is just asking for a short down the road.
So get a relay rated for the motors amps, and a push button switch with LED so you know when it is ON.
Find a key on 12v wire in the cab fuse panel, no 12v with key off, the fan will drain the battery if you forget to turn it off.
Run a small gauge wire to the switch and ground switch for LED, then small wire from switch to relay under the hood, ground relay.
Run the large fused power wire to the relay, then large wire from the relay to the fan motor.
You want a normally open(NO) relay
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