67 Dart Conv. - No Tail Lights, Park Lights, Flashers

If, with Key off, stepping on brake pedal changes the voltage in L8, I'd suspect the horn wires as they are always hot.
If this only happens with the key on, then it could be turn signal wiring.

Maybe time to isolate stuff.
I would say start with disconnecting the brake switch, but I know it can be difficult.
Maybe the instead disconnect the column connector. Not fun either but with patience less difficult.
This should take any short in the column out of the picture.
Then see if L8 works normally.