There's a switch at the base of the steering column. In fact there are two; IIRC one is the neutral/park safety lockout (stops you from cranking the starter in driven gears) and the other is for the backup lights.
They are adjustable to allow for slop in the column and shifter, and so the switch may be worn out or just out of alignment. Early ones like my '74 are screwed down with little sheetmetal screws; later ones I think just have plastic snaps.
I would find the switch, unsnap the wire connector and clean the terminals, and make sure it's fastened to the column tightly. If that doesn't fix it, then move on to adjustment and/or replacement.
-- A