2002 Chevy Silverado back up lights

Electrical problem
2002 Chevy Silverado V8 Two Wheel Drive Manual

How do you change the back-up (relay, switch, what ever it is called) on the transmission? My back up lights went out. I've checked fuses and bulbs. Mechanic said it could be the relay on the transmission. Been under the truck and see a couple on wiring plugs going into the tranny. People at Auto Zone weren't able to help me.I hope I don't have to go to the dealership and get bent over for a ten minute fix.I try to do as much as I can myself.