Dashlights flashing with right side blinker

[Aaron Wilson]

Hello, I have a 71 GMC 950, it has a Signal Stat 900 turn signal switch. When I turn the left signal on everything works good if I turn the right signal on with the headlights off the blinkers blink but so do the dash lights. If I turn the right signal on with the head or parking lights on the signal indicator and dash lights go dim and the signal comes on solid. I am assuming this is a ground problem somewhere. Is there a good place to start diagnosing this problem? I don't know what the thought process was but I thought I should add, someone has wired the taillights and the dash lights into a toggle switch on the left side of the gauge cluster. The factory headlight switch still turns on the headlights and front parking lights and also dims the dashlights. 

Thanks