[FirstOwner69]

Assuming you have an automatic...

There is a switch (neutral safety switch) mounted to the top of the steering column a few inches inside the firewall. There is a small screw on each end. You can adjust the timing of the backup lights by loosening the screws and rotating the switch from side to side. It takes very little change of position to adjust the lights. However, if one side works and the other doesn't, it does not sound like the problem is the switch. My bet would be on the ground. The switch also prevents the starter from being energized in any positions other than park or neutral.

The brake light switch is under the dash in contact with the brake pedal arm. It can also be adjusted fore and aft to change its engagement point. The single wire on the prop valve (or distribution block pre 1971) turns the brake light on the dash if there is a brake system malfunction.