MPI relay, starter relay, fuse, broken wire, there's a number of things between the starter and the key. To test, turn the key to the run position and put 12 volt power to the starter solenoid. (small single blade connector on the starter) my suggestion is to get a 4 foot piece of wire, put a blade connector on one end and put that on the starter, turn the key to the 'run' position and then jump the solenoid by touching the wire to the battery positive. This will engage the starter solenoid and turn the engine over which will likely start the truck. A less mechanical test (not making the engine run) is unhook the solenoid wire and put a test light on it, then have someone try to start the truck, if the wire gets power when they try to crank the engine over, the system is fine and the starter is bad. If it doesn't get power, there's an issue upstream (fuse or relay)