I'd say your right on track!! Also it could be the fan switch in the console!!If the 79 is wired anything like the 77 there could be two different possibilities. There is a relay bolted on the (center) right side engine compartment of the firewall. Unplug the connector and check the red wire with a meter or test light to a good known ground. If you have voltage there then the relay is bad or may be a dirty connection. If there is no power on the red wire of the connector then follow that red across the firewall down below the brake booster I think the high speed fan relay fuse is there. It is a 30 amp fuse that powers the high speed fan relay that you just checked.
Now these suggestions are from a 77 wiring diagram so I may be way off here.
Also it could be the fan switch in the console!!