Prairiepanda

I can't figure out why my radiator fan refuses to kick in automatically. The previous owner of the car had installed a manual switch for the fan, which works perfectly fine, but I was hoping to reduce wear on the fan by repairing the underlying problem to get it working automatically again (because otherwise I'm likely running the fan way more often than I need to)

I have replaced the fan relay, all relevant fuses, and the switch on the thermostat housing. The new relay and switch worked perfectly when bench-testing them prior to installation. I'm able to turn the fan on by shorting the wiring harness that connects to the thermoswitch, so in theory the wiring should be good all the way to the fan, right? And yet the fan never comes on even when the car is on the verge of overheating.

I still have the manual switch installed, so I'm using that for now. But I would really like to fix this.