Why is my truck not shutting off?

You didn't state year, make or model so I'm going to make a guess here. This sounds like your getting feedback through the alternator fields that keep the ignition circuits energized after the key is turned off. Try electrically disconnecting the alternator, start the truck and then see if it will turn off as it normally would, if it does then replace the alternator as you have bad diodes inside the alternator. If it still does it then you have a cross-circuit somewhere in the main harness that has made contact directly to main power. And that is going to be a nightmare to find.

answer not posting

Make , Model , Year ? ? I'm thinking maybe the switch has a problem and is letting juice to back feed and power the coil . That's my impression .

Ita 2003 Chevy suburban 5.3l 15002wd I changed ignition switch an dimmer switch neither one fixed the problem