69 Dart short

OK first thing is pull apart the LF lamp and inspect carefully.........short (cross) between TS wire and park wire, and "if not" wiggle the heck out of stuff. With bulb installed this WILL show as a short so next step is to pull bulb and see if "short" goes away THEN SEE if the bulb is actually shorted internally or if it works. So jumper bulb "alone" to power source and make sure each filament lights individually to the shell

Next check out rear harness when disconnected. Pull bulbs and inspect as above, and with bulbs removed check for harness shorts and crosses between circuits. with stop lights removed, stop lamps should show individual continuity from one end to other BUT SEPARATE (infinity) to each other (left vs right). and so on.

Sounds like you have two problems