1973 Duster Heater fan not working

Great. I was looking at the wrong part of the drawing. In the lower part there's a heating and air conditioning blower motor that caught my eye and I went from there. So let me rework this. According to your drawing the black wire going into connector CI 41(pictured in your photo) is carrying 12 volts from the fuse to the switch. The red wire is supposed to carry it on from there to another destination, but as it's been cut it is no longer doing that. The brown and dark green wires pictured in the same photo then continue on to connector CI 33 at the resistor block. At that connector the dark green wire passes through providing 12 volts to the motor for high or a reduced voltage through the resistor for low. A black wire coming from the blower motor also passes through connector CI 33 on its way to ground. Using your volt meter with power on and the switch in the high position you should be able to see 12 volts available on the blower motor side of the dark green wire coming out of that connector. If it’s there you can rule out the switch or power supply as the problem. From there confirm that your ground is good. In other words, with power off check continuity between the black wire at connector CE 33 and ground. Loss of ground can render the motor inop as well. If you have power and ground the issue is the motor. If you don't have power there the issue is somewhere up stream.