The good news is someone out there has a page on how to do this. The bad news is I no longer have the address.
But it goes something like this FOR A 1985 K100. Remove the tank to access the fuse box. Locate the fan relay. Pull the harness block with the leads off of the relay.
What I call the top will look something as follows. I am not sure if this is looking at the relay or the connectors on the harness side. I think it is connectors on the harness side but you get the idea.
If you turn on the power and run a wire between 15 and A2 on the harness side, the fan will run continuously. So, you must run one wire from 15 and one from A2 with a switch connecting the two.
As I dimly recall, I did this by using what I call the bottom, the side of the connector block with the wires from the harness:
I made two Y connectors. First, I pulled out the 2 Green/brown, attached one end of my Y to it, and then put one end back into the connector block. Second, I did the same with the Purple/Yellow. Pull out the Purple/yellow from the connector block, connect this to one end of the Y, and put another end of the Y back into the connector block.
I am adding two wires to the harness by splicing but without cutting the original harness. Connect the harness block to the relay. The fan will now run as original, but I have two wires running out of the harness from the motorcycle. If I connect them when the key is on the fan will run. Run those two wires to any desired spot, add a connecting switch and you are done.
You can make the Y connectors by crimping if you can't solder. You will need male and female connectors. I managed to blunder my way thru; so, you should be able to also.