I have just tail light on drivers rear no brake light or blinker I have replaced bulb,socket and multi function switch all fuses are good any ideas ? Also think might be ground but I don't know where grounds are or the wire harness that runs from cab to tail light

 

I think both filaments use the same ground, so if one of them work... both of them should. Can test that theory by having someone step on the brake, and test the bulb socket for voltage, and just ground to the bumper, or something else convenient (metal...) back there. Might be a bad multi-function switch.