76 f-100 heater, a/c problem

I have a 1976 ford f 100. The heater, a/c switch does not turn the blower motor on. I have replaced the heater relay. If I jump the the relay, the blower motor works on all the differant speeds.
Does anyone know where I can get a heater, a/c schematic? Has anyone ever come across a problem like this?

Here are some diagrams that arent great, but maybe theyll help:
http://autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/0c/e9/82/0900823d800ce982.jsp
Youve got my curosity up about that relay. Could it be one that someone added ? Ive never seen one controling the blower motor on a truck of that era. Ive installed them on some of my own trucks, to cut down on current flow thru the switch. I have a 75 that doesnt have one, and i cant find it in any of my schematics or books. Are you talking about the circuit breaker for the heater/ac maybe? If it is a relay, and you can jump across it and make the blower work, youll need to find which wire energizes it, and trace the circuit . Maybe you can see something on these drawings that i cant, im half blind anyeway. lol There were a few years on some of the trucks and vans with ac, that had a control unit behind the seat. I think they quit using that in 74 or75 though.

Got it working. I got a schematic and did follow the wires. I just bought the truck and the heater, a/c fan didn't work. So I found someone did not have the wires hooked up properly. Thanks for your replies.