Electric fan control confusion

[87chevy.com]

Originally Posted by NC_John

I don't think thats my problem. I'm getting a fan-on 12V signal. For some reason, the relay just won't engage.

How do you know your getting 12v fan signal?
How are you measuring it?
The pcm supplies ground to the relay. The + wire is a strait power or ignition depending on how the harness was made. I would measure battery post - to the " + " from the harness and see what you get. Try no key, key ignition on. Do that for both " + " wires. Then battery post + to the wires marked " - " and then use hp tuners or equivalent to activate each one to verify pcm is giving the on signal.
Post up what you find.
Clint