To wire a thermostatically-controlled oil cooler fan, such as the '87 and later Carrera on an earlier car, you will need a relay socket (about $10 from a dismantler), a relay, and a rear wiper switch (optional).
1. Examine the relay and socket to determine which relay pin sockets connect to which terminal on the relay.
2. Wire terminal 30 of the relay socket to the bottom of fuse 13 (counting from the front of the car, in this case an SC. This fuse is a 25-amp, for the sunroof, rear wiper, and mirrors.) Use heavier gauge wire, such as 10 gauge.
3. Wire terminal 87 to the fan motor positive terminal, using 10 gauge wire.
4. Wire terminal 86 to the bottom of fuse 14, using 16 gauge wire (16 amp fuse, wiper/washer and cigarette lighter.
5. Wire terminal 85 to the thermostat, using 16 gauge wire (if you want to include a manual switch, also run a 16 gauge wire from terminal 85 to one terminal of a switch, and connect the other switch terminal to ground. A rear wiper switch works well for this, and can be located in the stock position. On many cars, the hole is already cut in the metal dash. You can locate it by pressing on the dash along the left of the steering wheel below the instruments. Cut out the vinyl with a razor knife, and the switch snaps into place.)
6. Wire the fan motor ground terminal to ground.
Before buttoning everything back up, confirm that the fan works properly and that it blows in the desired direction. It should activate when the ignition is on and the manual switch is closed, or the thermostat should turn it on at about 248F.
This is also a good time to ensure that there are no shorts from incorrect wiring or faulty circuits. Make sure that all the doors are closed, and all lights are off, including the boot light. Disconnect the battery ground cable, and connect a test lamp between the ground cable and ground. If it stays lighted, you have a current draw. Remove one fuse at a time until the test lamp goes out, and you have found the bad circuit.