65 dart turn signal wiring question.

Is there any possibility the TS switch could be wired wrong, etc? Again I suggest "if you have not" to check the bulbs carefully. Either wrong bulb or one that is internally defective

It IS POSSIBLE the TS switch can be bad as well. You might disconnect the TS switch and check it in each position, make a "truth table" and compare, so like this:

First, if you have (a separate switch) disconnect it and recheck vehicle lamp operation

Next disconnect the TS switch and make a chart from the diagram of the wires identification

Here is what you have

With switch centered, the brake light wire should go to both rear lamps wires
No other switch wire should show continuity to any other. Just those three

With switch on the left, the brake switch wire should go to the right rear lamp. Check all other wires to make sure it is not "also" connected to them

In that position, the flasher wire should connect with both the front lamp wire and the rear lamp wire

this is all reversed side-for side with switch in the right turn position.

Again, check this with the TS switch disconnected--so that a cross in the harness will not affect it.