My sons is having trouble with the #14 fuse.

Why were those parts replaced? Was that in an attempt to solve the problem, or did the problem start after they were replaced? There will never be a fuse in the starter circuit. Some cars do use a very large fuse for the generator but it will usually be bolted into the fuse box and will be around 100 to 120 amps. What is the rating on the fuse that's blowing? Is it inside the truck or under the hood? Which systems don't work when the fuse blows?

A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a 12 volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it's not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.