1993 Toyota Celica GT 150,000miles... lights flicker

I recently replaced the headlight assemblies for this car after an accident. I spliced the wires. This was fine for over a year and half, now whenever I press the brake pedal, the dash lights, headlights, etc. dim almost completely out and come back on. Also noticed a flickering of the dash and headlights while driving. Is there a loose connection or something else like a headlight switch (NOT EVEN SURE IF THIS HAS A HEADLIGHT SWITCH) Any info would help. Thanks