Oddly enough I was doing a little trouble shooting today on my carb fan as well. It's the only thing not working since I swapped the harness.It's the California spec 76-77 carb cooling fan. I think in 78 all 40's got the cooling fan but it moved to the left fender apron where those louvers are. On mine that's where my windshield washer fluid reservoir is but that moved to the other side of the engine bay in later models. The fan works and my sensor works but it hasn't ever kicked on. I've tried 2 different controllers too so I'm going to have to bribe Senor Ed to see if he'll help me trace it down. The fan is big and ugly and I love it. I just need to get it to work. What might be happening is since I swapped out the Thermoreactor exhaust manifold for a Fed spec manifold it's not getting hot enough to trigger the sensor to close the loop. I'll need to get an IR temp gun to see about that and then see what temperature the sensor closes with a pot of water and a thermometer.
You might check to see if you are getting 12V at the G/W wire (ignition off) on the controller connector.
It looks like the carb fan was a USA "add-on." On both my harnesses, there is a in-line fuse holder that's about 6" from the controller connector on the G/W wire. It was wrapped in yellow tape. I think the schematic shows it at a 5A fuse.
I'm hoping it's the culprit for my fan not working but I haven't got that far yet.