No left brake light only when headlights are on

[rbbc]

I am having the following issue and would appreciate some insight from anyone that has dealt with this in the past. With my headlights or running lights off, my hazards, turn signals and brake lights work, but the left brake light is dim. My turn signals also blink at the same speed. With the headlights or brake lights on, the left turn signal blinks slowly and the left brake light does not come on at all. I have replaced the turn signal switch (twice) and brake light switch, checked the connections under the dash; gone through the grounds for the taillights (wire brushed ensuring metal on metal contact), running lights, headlights, and battery; and inspected the harness from the firewall all the way to the taillights...all to no avail. I did notice that my truck is missing the big ground straps from the valve cover gasket to the firewall, from the cab to the frame, and from the bed to the frame. I have ordered those and will install them to see if that helps. Has anyone had any luck fixing this type of issue doing something different than I have done above? TYIA.