Assuming that '72 is the same as '74, then yes the switch is in the ground side of the circuit.
With the switch in one position the map light is connected directly to ground through the dash frame and turns on.
With the switch in the other position the map light is connected to the yellow wires of the dome light circuit, so the map light comes on when the doors are open or the headlight switch is turned.
On my '74 the hot wire for the map light is pink, the same as the hot wires for the dome light, trunk light and the glove box light (the other lights with ground switches) and the ground wire from the map light to the switch is brown, and the wire from the map light switch to the dome light circuit is yellow (the same as all the dome light ground wires.)
