"...with a short between brake and tail sending 12v back into the tail light ..."

There is not a "short" between brake and tail light. The filaments are connected to a common ground terminal. If the bulb is not properly grounded the 12V travels across the tail light filament seeking a ground, which in your case apparently it found it after traveling to the switch and the dash light circuit.

"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.