I've done this on my 91 but it uses switched 12v not ground.


This is a rough wiring diagram of what you can do if you want to use a relay and a switch.
85 Fused switched 12v or ground
86 Fused constant 12v or ground
30 A/C fan
87a Factory A/C fan wire
87 Ground

You can either used switched gound or switched 12v to activate the relay just keep in mind that if 85 is 12v than 86 has to be ground, and vice versa. Fuses are your friend don't be afraid to use them, the whole point of using a relay is so you don't draw a bunch of current through a switch, typically a relay is rated for 30 plus amps, when your $3 switch from the store can handle 15 or less amps. Also using a relay this way allows you to manually control the fan when you want to and also allow it to work automatically(factory).