Swapped to Muscle - 75 Duster Daily build

The backup light shouldn't cause your issue.

So, looks like power for the turn signals comes from the #2 fuse. It splits a couple of times but in regards to the turn signals it hits the flasher through a black wire and leaves the flasher on a red wire (see A27-28 of the wiring diagram). It looks like you already tested that, so I would go back to the fuses and make sure #2 hasn't blown again or doesn't still have an issue. If the fuse is still good, check your blower motor and see if it works. If so, then power is getting that far. The next place it splits is at the radio. If the radio works or the red wire with the white white spliced at the plug has power then your issue is still downstream. The next split is the one you found already in the image above.

Not sure what the plug with brown and yellow wires is for, there are only 2 flashers in the diagram for a '74 (turn and hazard) that I could find. Maybe '75 has another one?

BTW, fuse #4 looks to power the AC circuit. If you don't have AC, it is probably empty.