1973 Duster Heater fan not working

ok, let me test these things with the car running and see what I get. Also, yes I have a rear defogger, but have not tested to see if it works, as I live in Las Vegas and would never use it. :)
Also, quick question on the wiring diagram: so in the red circled (for example) it says C4 I4 BR, I get that the wire is brown, but what do the C4 and I4 (eye 4? or 14?) mean?
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