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That seems like it would eliminate any circuits outside the cab. Try unplugging the brake light switches and jumping past them to see if that changes anything.

Possibly the stop light switch output wires will have continuity to ground through the lamp filaments.

Maybe the circuit breaker is too small? People love to move relays and CBs around without regard for where they belong.

Unplug the turn signal switch and see if the breaker still trips when you hit the brakes. I think that is where the brake light feed is interrupted so you get directionals when brakes are applied. Could be a short in the switch.

Maybe reconnect one wire at a time to the brake switches and see which wire causes the short? Maybe one wire engages the exhaust brake and the other the brake lights? Just guessing, sorry, my fmtvs are older.