the switch gets it's ground from being mounted to the metal dash, but mostly the lights only get power from the switch, and ground from their socket mountings
for the dome light, there should be one wire from the main light switch to the light, which is hot - the 2nd wire comes back to the switch and attaches to the variable resistor on the front of the switch which supplies the ground when the switch is rotated - in the off position the circuit is open - if you are measuring "hot" at the light, you're just measureing the same wire on both sides of the bulb, of the light doesn't turn off with the switch, the switch may be bad
for the brakes, power from the light switch goes to the brake switch, then straight back under the truck to the tail, and the tail ground is via the frame to the light assembly - nowhere will the dash light wiring intersect with the brake wire - got a shop manual? there's a wiring diagram in it
Bill
