[dscott]

The original Classic harness used wire # 151 for fan ignition feed. This wire went to a Black Magic fan switch which had a built in relay. If you have this wire supplying power to the fan it will melt or burn. You could be trying to draw up to 30 amps on a 16 guage wire. Use a universal relay and wire it as follows.
On the bottom of the relay it is numbered 30,85,86,87.

Run a 12 gauge wire from # 30 to the fan +. Ground the - side of the fan to the chassis.

Run a 12 gauge wire to # 87 from the battery or the firewall circuit breaker.

Run wire # 151 from the Classic harness to #85 on the relay.

Terminal # 86 will run to a fan temp sender on the engine (you will need to buy one). Jegs or Summit can supply this.

If you want an over ride switch on the dash then run a wire from # 86 to the switch or use # 157 from the Classic harness.

On the dash over ride switch you will have wire # 161 and # 157. Remove wire #161 from the switch and tape the end, it won't be used. Run a ground wire to the switch terminal that wire # 161 was on.

Terminal # 86 is the ground for the fan circuit, the temp sender and the switch both supply a ground when either the stated temp is reached or you turn on the switch.
Don