If it's like the 71-74 gauges, they all ground as a unit (except for mech speedo and ammeter which has no ground).

When you say 'brake lamp', do you mean the light that lights up when you have the e-brake engaged? If so, that should have a wire that connects it to the parking brake switch which will connect to ground when the parking brake is engaged.

I would pull the gauge cluster and verify the voltage regulator (little box on back) is well-grounded...should ground to dash frame. The parking brake light socket should have 12V when the key is in the run position.

A wiring diagram would help here...