That will be the inhibitor switch to stop the truck starting in gear. Start it up put it nuetral turn it off then start it again... if it starts then there is somthing shorting that circut
Check the wiring to the inhibit switch on the auto as that fuse powers the reverse light switch part of it. Also have a good look around the reverse lights and wiring at the back. It is uncommon for this issue to happen on an auto it is normally the manuals that get reverse light wiring shorts.