1973 Duster Heater fan not working

I forgot to mention, unless your car's been modified I believe the blower motor only works when the ignition is in the on position. If the ignition is on you should have power at the fuse. If you don’t there’s no point looking downstream. The problem is between the fuse and the battery somewhere. I notice that the same wire that brings power to your heater blower motor fuse also brings power to the rear window defogger system fuse. Does your car have that system and is it working?