Dashboard Brake Light Comes On. Why?

Go to MyMopar. Download the 72 Plymouth service manual. That manual got there from a guy right here

There are only 3 things that can turn it on

The switching is done in the ground leg, IE the lamp gets power on one side of the lamp, and the switches are on the other leg to ground

1...There can be a wiring short to ground in the switch wiring
2....As mentioned earlier, your parking brake switch can be on/ grounded
3....The differential brake pressure switch can be activated