[jhow66]

Check the large red wire at fuse panel. It plugs in to a socket labled ing. This wire supplies current to the relay on AC box beside glove box. With switch on you should have current to relay at the red wire. (with blower switch on high it closes a set of contacts sending 12 volts to fan motor).If no current at relay trace wire back to fuse panel. You probley will find a loose connection somewhere that has gotten hot enough to make a bad connection. ( Fixed a 71 this past week that had the exact problem you have--it had the above problem). Hope this helps.