If the ground is switched, you could cut the ground wire you mention, solder in a wire going to a switch that is connected to ground and then heat shrink the soldered connection. Pretty easy to do, and you retain normal functionality of the fan while adding manual control.

HTH


73
-Jon
KJ6GVM

As seen on Expedition with TX plates: VEGETARIAN - An old Indian word for poor hunter

Grampa's Trooper
1974 FJ40
1987 FJ60