Sorry. I was looking at the Wiring diagrams for the 1999-2004. The older Mustangs have a single speed fan.
For the 1999-2004, the V6 fan has a LOW speed only. The V8 has a two speed fan. The v8 fan uses all 3 wires. Low speed is RD/OR. High speed is OR/LB. Ground is BK.
The V6 fan has RD/OR and BK only.
The V6 fan will work fine when the PCM calls for low speed. When the PCM calls for HIGH speed, the fan will not run (unless rewired). Expect to overheat on a hot day stuck in traffic.
You hit on something Fox...When I swapped my old fan for the new one my car ran a ton cooler. I think the newer fans have a better design allowing better airflow.