'74 Dart Sport Wiring Diagram Problem--HELP!!!

You should get yourself a reprint factory manual, either paper or CD, or both. The wiring diagrams for 74 were kind 'o a mess. I can post sections of it by photographing a printed manual I have.

There were no chronic troubles of this specific kind, necessarily, that is, you need to take the problem as a "one off" and chase it down.

This is how I read it.

From the fuse, a no 14 BK feeds over to the heater switch connector,

and branches, runs off as no 14R to the radio power connector,

branches, and runs off as no 18W which branches off to backup lamp switch, (through firewall)

branches and and runs on, no 18 bk, finally, to the turn signal flasher.

So there's a lot of "stuff" on that fuse and what essentially is one wire.

ANY of those items on the windy route could have cause "meltage," the melted area is at the LAST of the run, you can rule out anything before the melt area.