Ok... we need to be "clear"....
You (maybe) want to do this for the front fender mounted oil cooler fan? Your text is not very clear on this. Some folks might think you're talking air fan on the engine at first.
I've done this. There are a number of ways to go. I don't have the capability to upload a wiring diagram, which would be the best, so I will try text.
You need to understand electrical circuitry. Pardon me if I'm wrong, but I'm not 100% certain that you do, based on your text. Doing something wrong electrically can be a big deal, so excuse my less-than Emily Post etiquette.
A relay "pin-out" is fairly generic. #30 and #87 are the end-points of a switch that opens/closes on the main power circuit. Try to visualize a knife-switch that has a handle on it so you can open/close it by hand ( you'll see where this is going later). The other pair of contacts....#86 and #85 are the end-points of an electromagnet that when energized.....act like your "hand on the lever" and close the contacts 30/87. So the 86/85 pairing is the "control" circuit and the 30/87 pairing is the "main power" circuit".
here's something that helps visualize a relay-->
http://electronics.howstuffworks.com/relay1.htm
So now you know the various contact points on the relay.
I wired my system this way:
- at #2 ( 16A) fuse of my 10 fuse block on my 85 Carrera....took wire from the output ( bottom) and fed to #30 on relay. ( This is existing power from battery thru a 16A fuse). 1987 and 1985 Carrera's use similar (maybe even identical in this area...but CHECK) fuse boxes.
- fed wire from #87 to positive (+) connection on motor. Fed wire from other motor connection to ground.
- I decided to fuse the "control" circuit" and not JUST the power circuit as Porsche does stock for the 87+ cars. Found a wire from ignition switch to fuse #5 on my block. This has a common bridge to fuse positions 4, 5, 6. Number 4 is lowest amp fuse of these three ( 8A)....and I fed a wire from the output of #4 ( bottom) to my dash mounted switch ( Dorman model 643-224 that has an amber light)...connection #3 ( labelled "power" on the switch). Connection 1 (labelled "ground) on the switch went to ground. Connection 2 on the switch ( labelled "accessory") went to connection #86 on the relay. I fed a wire from relay #85 to ground.
Something along these lines may work for you too but you are advised to find a model-year specific wiring schematic for your car. Another point...the relay "pairings" are fairly generic, but are bi-directional. Pairing #30/#87 might be interchanged....and #86 /#85 might be interchanged. Doesn't matter.
Hope this helps.....draw it out..it'll help in the visualization.
Doing it "this" way...minimizes the "amp" draw on the switch. The switch is not "directly" part of the power circuit...as it seems you wanted to do. My switch only controls the relay's electromagnet...which is orders-of-magnitude less amp draw. The relay power contacts, then "act" as the switch. Most relays have a 30A rating..plenty for the fan and wiring.
- Wil