If you don't have a high current alternator, you can forget about using an electric fan. The stock 65 amp alternator on 86-93 Model Mustangs isn't big enough to run the fan and the rest of the car.
3G 130 amp alternator installation instructions: Stangnet 3G install sticky
3G Alternator Install: A How To - Mustang Forums at StangNet
The 30 amp fuse is suitable for use with 10 gauge wiring: do not oversize the fuse rating since you risk an electrical fire.]
Wire size current table:
18 gauge wire = 5-8 amps
16 gauge wire = 10-12 amps
14 gauge wire = 15-17 amps
12 gauge wire = 20-25 amps
10 gauge wire = 30-40 amps
8 gauge wire = 50-60 amps.
If you are good with electrical stuff (90% of the people here aren't), build your own controller. The numbers on the diagram (#86, #87, etc) refer to the numbers on the bottom of a typical automotive relay.
The connections for the recommended Bosh relay will probably be identified differently.
Note that the thermostat switch in the diagram isn’t really suited for the job. You’ll have to do a Google search and find your own.
This version of the fan controller will continue to run the fan after the ignition switch is turned off, just like most new cars. The notes about the Bosh relay, wire size and fuse ratings still apply.
To allow the ignition switch to control the fan so that it does not run when the ignition is off, connect the relay contact #86 to the red/green wire on the ignition coil or to the red/yellow wire on the coolant level sensor.
Alternate placement for a temp gauge sender or temp switch/temp sensor for an electric fan. Use the heater feed that comes off the intake manifold. Cut the rubber hose that connects the manifold water feed to the heater and splice in a tee adapter for the temp gauge sender. Be sure to use the same water feed line as the ECT sensor. That way you will get the most accurate temp readings.
Tee adapter info:
Make a pilgrimage to your local hardware or home supply center and get some copper pipe and a tee that fits the temp gauge sender. Solder two pieces of copper pipe onto a copper pipe tee with threads in the tee part. Find the correct brass fitting to match the temp sender threads to the tee fitting.