Electrical problems with 78 sierra grande

[jacob13]

Need some help new to this.
I have a 1978 GMC Sierrra Grande when i first got it everything was working fine. First when i would turn my right side turn signal they both would flash at the same time just in the rear . Then a couple of days ago my tail lights stopped working. I've replaced the bulbs and checked the fuses all ok. The front signals and marker lights all work the back signals blink and my brake lights work. I checked the plug where the rear wiring harness hooks up and nothing when i put on my blinker i get 12 volts any ideas what it could be would be appreciated.

Thanks jacob13