68 Driver side rear light issue

[bretcopsey]

Need some direction on what to check for this problem. The driver side rear running light works as it should, but second filament doesn’t flash with turn signal or 4 way, nor will it light with brakes applied. Front park/signal works fine and passenger side all operates correctly.

I hooked up some trailer lights in place of taillights with exact same results then swapped the trailer light wiring to oposite sides and the problem followed to the passenger side so this tells me there is some sort of short upstream in the DS circuit (under dash?). Since it’s both brake and flasher I’m hoping this narrows down where I need to look. Bulbs are good and both flashers have been replaced in the fuse block. All fuses look good and I wouldn’t suspect that to be a problem since it’s just one side with an issue. Any tips would be appreciated.