Single speed? Dont know. (maybe that is what the little black box does)

here is what I do know. Maybe you guys can help me identify it.
The relay is pluged into a little black box, with several wires comming out of it. It (the relay) was jumpered and cant trust how it was wired in, made no sense to me.
The little black box had some electronics in it with a pot showing( could be adjusted with a small screwdriver). 2 very thin wires came from the thermostat in the manifold to this little black box along with a couple hot wires (best I could tell) and a couple more comming out.
Does any of this make any sense to any of you?

Today I ran both fans into 10g wire all the way to the fuse box area. I am prepaired to make all the connections in that location. I tested the fans and they pulled 11 amps when running full speed and 21 amps starting up.

30 amp relay should do it? That is if I cant figure out how to use the existing relay, and that little black box.

What do you guys advise? (dont say sell the truck)

Thanks Jim

and your right Shawnlee, much longer and a bit more.