You did good and you are doing good. You are as capable as any of us. You will laugh when you find what is wrong. Your overall plan is spot on.
Now that it "seems" like it is the switch, I guess you will start with the three left turn wires/pins to see if they have continuity. In your case you know that one of those sets, 12V turn sig feed and rear left lamp wire, have continuity because you said the rear left blinks.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn