Your park lights have nothing to do with your signal and brake lights other than a common bulb but on the other filament. if your signal lights are working at the rear of the car your tail lights have ground , your wiring is not an issue on the brake light/signal wire as you have proven that already.
You have two separate issues
park lights

brake lights

They are different circuits going to the rear of the car . do you have park lights on the front ?
I would give no credence to your wiring harness codes as your harness could have been altered.
It should be very easy to confirm your park light circuitry by unplugging from your head light switch ...finding the hot wire and then jumping to the supposedly correct wire as the book says.
If that doesn't work , go back to the trunk and unplug the tailights and jump in with a known 12vs.
remember the park lights are the dull side of the bulb.
Now your brake light,i suspects its the harness plug at the steering column providing you already proved that there is always 12v at the brake switch.
there are not many more things it can be .....its not the bulbs , its not the wires going to the rear. so whats left?
Power to the brake switch , steering column harness plug or the signal switch . nothing else to look for !