Left turn equals both tail lights flashing???

[HSRACER]

So I just got a new to me 69 gmc and I was inspecting the lights before driving it home. Everything works properly until I put the left turn signal on. The front flashes fine but in the rear both sides are flashing. Right turn works properly with only the right side flashing in the rear.

I pulled the connection at the rear and found both lower pronges had flashing power to them when the left turn was on, which means that the problem is up stream of the actual tail lights and rear wiring harness. The factory leads along the frame are all there and connected and the plug at the firewall is solid and correct. It then seems to go to the steering column at the lower connection point at the bottom of the column, which I thought was neutral safety switch?

Any advice of what to check next is appreciated!