Fuse blows every time I turn the key to on

Every time you turn the signals on, or every time you pop in a new fuse? Are you in the fuse box under the hood or inside? Have you checked for damaged trailer harness wiring? That is a common cause of blowing fuses.

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 twelve 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 is 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.

The trailer usually is not the issue. Problems are almost always with the wiring harness on the truck. They get chewed up and short together.

I will be back tomorrow to continue. I am freezing in my van right now and I want to look at a wiring diagram so I give you the right information.

I could not find a drawing of the fuse box layout, but I did find the drawing of the wiring connections that should get us to the same place. See if you can find the naughty fuse on this drawing, then I can find it on the schematic diagram to see what is involved. You will need to find two numbers on the drawing corresponding to the fuse's two terminals.