It sounds to me like a defective switch. There should be no flashing when the lever is in the center position. There is power to the flasher at all times (when ignition is on) but there should be no path for it to travel when the switch is in the center.

The extra ground wires were a good idea but you also need to be sure the body and bed are well grounded to the frame.

You are mistaken in your thinking the tail and brake light filaments are different voltage. The difference in brightness is accomplished with different size filaments.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.