There is a yellow light on the tach face, alongside the red overtemp light, which was connected to a switch on the choke cable to illuminate when the choke was engaged, on early model K-bikes. Later the choke switch was removed, but the light is still in the instrument pod, and the cable is still in the harness.
I did some wire checking on my K75s. Under the front of the tank, on the left side of the frame, is a two wire cable labeled "choke". The connector is about 3/8" flat, with a brown and brown/violet wire. It is free (not plugged in to anything). I was able to access this area by sliding the tank back. I didn't have to remove the tank. First, short the two wires together with a jumper (I used a chunk of solder, folded into a "u"), and turn the ignition on. The yellow light should be illuminated. If not, perhaps the bulb has been removed, or you have the wrong wire. Pull the jumper, and the light should go out.
Locate the wire going to the fan. This is a heavier connector, 5/8" flat, with three positions, but only two wires, a brown, and a violet/yellow. From the right side, down low, I could see the route of the wire up from the fan to the area where I found the connector. Both of the connectors are in the same vicinity, making wiring easy.
The light is expecting a contact closure from the old switch, so I connected those two wires to the Normally Open pins of a Radio Shack "12VDC PC RELAY" 275-241.
I wired the relay coil in parallel with the fan motor, leaving the original fan power wiring intact.
I covered the relay completely with RTV silicone (it's only about 1/2" cube), and taped it, then laid it on top of the ignition module.
At an ambient temp of 40 degrees, I started the bike and let it idle for 10 minutes before the light came on. I couldn't hear the fan, due to running the engine at about 3000rpm in a carport. When I dropped the bike to a normal idle, I could hear the fan. At an idle, the fan shut off after a few seconds. Oddly, the light stayed on for another second or two after I heard the fan shut off.
Now I have a yellow light indicating fan activity.
I really wired the light just because it is there, but it is a good indicator of heating when you can't hear the fan, or as a clue that a fan has gone bad.