Good point about soldering connections. Thats the way to do it.
Thats good to know about the electric fan. I'm currently running a flex fan and would like to switch. I'm going to have to do some looking and see if I have room. I think my radiator is about 3.5 inches thick. It looks like I have about 3.25 inches max between the radiator and the water pump pulley.
It's a coolant temp sensor that is actually immersed in coolant, just like the one that feeds your temperature gauge. It sounds like skywalker used a thermostat housing with the threaded hole to add the additional sensor to his engine.
If you look at the fan controller part number on Derale's website, there's a picture of the part.