[RIP]

I don't have your manual but I'd say power at pin 31 confirms the circuit from the ignition switch through the relay to the PCM is good correct?
I'm not sure there's a safe way to check to see if the PCM itself sets the ground. Sounds like you have checked all you can. Now comes that decision to spend big bucks on a part when you're not 100% sure it will fix it. I've been there and feel your pain. Unfortunately, it can come to that. I'd recommend you first check local junkyards to get the lowest possible price. I would expect less than $100. Then comes Ebay at around $100 -$150. Then the internet sites at around $250. Your dealer, if they can even get one for a 92 would be in the neighborhood of$300 -$500. A dealer PCM would have the latest updates.

I've read once or twice about people jerry rigging a switch to bypass the PCM. Don't recall how but it may be an option.

Just to make sure, the fans aren't coming on either by engine temperature or with the A/C switch correct?