The harness / wiring for AC / non AC is completely different.
I'm not sure if 73 wiring is same as 74 but you can now download a 73 shop manual towards the end of this thread:
http://www.forabodiesonly.com/mopar/showthread.php?p=1970088617
The simplified wiring for the A/C blower is page 24-45, that is, in the heater / AC section
Wiring diagram index is page 8-153
Notice that the blower motor references for heater blower and AC blower are different (D36 for AC and A36 for heat only)
If you go down to reference A36, you can see the heater wiring. This is all INSIDE the car, not in the engine bay.
If you drop down to D36, you can see that the dark green goes through connector CE2 ( Z ) This is the bulkhead connector. This means that on AC cars, the blower motor wire is IN the engine bay instead of inside the car
Go to page 8-164 and find the diagram of CE-2 and Z is where that wire should come out for AC cars.
Again, I'm not SURE that 73 and 74 cars are the same. Because you are mixing AC non AC and 73 / 74, you might have to do a little "diddling" to get this hooked up
