Backup light actuater

When I place my 91 Blazer in drive the backup lights come on. Where is the actuater? What makes them go on and off? Any help?
Thanks

I've started another thread asking about the selector lights on the dash for PRND23. I'm not sure but the Safety Neutral Switch on the side of the transmision tells the computer when you are in Park or Neutral. Otherwise you can't start the car. It may provide the feedback to turn the backup lights on. There is an adjustment on it.

Can't find your post. I have a 91 full size blazer. I have also found alot of help here.