There is an adjustment underneath where the switch is bolted onto the bracket.
Make sure the actuating rod is doing its job when in reverse and out of reverse. No rod? I made one from a metal clothes hangar.
The switch has slotted holes for adjustment. Back your car up to the garage door at dusk, jack up the side and place jack stands underneath. Turn key on, take 5/16 inch wrench, crawl underneath and adjust for lights off in neutral. Check to make sure they come on in reverse. Job done.