If you want some help you are GOING TO HAVE to provide some info.
THERE IS NO FUSE!!! Certainly not a 20A fuse
So "let's start again?"
WHAT IS IT that you are working on? Year make and model?
EDIT............74...........sorry see that now............
IS THE WIRING factory or modified, to the best of your knowledge?
WHAT IS this 20A fuse you speak of? It seems to have been added
Look......WE are not there.........WE cannot see what you see. No idea what is going on without more information Several of us here KNOW these cars and KNOW how they work.
Someone already suggested a melted wire in the harness. That is a good possibility.
If some jackass has modified the wiring, several things are possible, including a backfeed from somewhere to the starter (yellow) wire which fires the starter relay
Added EFI, non factory electronic ignition, etc, play a part in this
What do you have for wiring diagrams?
You can download the 73 service manual from this thread........
Free service manuals
THERE WILL BE some differences between 73 and 74 wiring
Also from Mymopar you can download the same manual (which came from here LOL) as well as AFTERMARKET wiring diagrams which "can be" easier to follow BUT they sometimes leave out details and options