[Dave at Pelican Parts]

Check the wiring diagram for your 73. (In Haynes, also available in various places on-line.)

Component #30 is the fan switch, which is what you are referring to. It has seven wires that connect:
- Red/white (red with a white tracer stripe) that brings power to the switch when the key is on
- Black/blue, 2 connections that bring power to the lights inside the unit when the headlight switch is on
- Brown which is always grounded
- White/green which is ground for the fan on slow speed
- White/yellow which is ground for the fan on medium speed
- White which is ground for the fan on high speed

The fan operates by having power sent to it at all times when the key is on. The switch provides the ground, with more resistance for low speed and lower resistance for medium speed--and no resistance for the highest speed.

Your wiring has been messed with. Fortunately, it looks like whoever did that at least tried to follow the factory wiring for the most part. Those three white wires have aftermarket connectors on them, and rings are painted around the wires instead of stripes. Still, they look pretty close to yellow and green rings, so that's good, and they're in the right place for the fan grounds--all right next to each other.

I frankly can't tell what those remaining wires are plugged into. And I don't have time to dig through my stash to find my spare switch. Unfortunately, it looks like the dark-brown one has white rings painted on it, but there aren't any brown/white wires that connect to the switch on the diagram. Those are typically some sort of switched ground connection, BTW.

You can take an ohmmeter or a test light and check if either of those two mystery wires gets connected to one output wire when the switch is in the correct position for it. That would be the ground.

Good luck!

--DD