Check the switch. It might have a short. You can eliminate it from the circuit (unplug it), and turn the truck on again. If it doesn't blow the fuse, there you go - probably at or very near to the switch - or the switch itself. It could still be after the switch - so between the switch and the motor, but this is a very good start.
If you still blow a fuse with the switch disconnected, check between where the switch is, and down to the fuse panel. Look for bare wires, etc.
If this simple step doesn't work, then you'll need to separate each part of the circuit, and run continuity tests. This is the most thorough, and appropriate way to do this, but it's going to take time and patience.
I will also add this important tip - DO NOT DEVIATE FROM YOUR WORKFLOW. People will have all kinds of ideas. They are not there, and they frankly don't know (including me!). You will have to play Sherlock Holmes and do this correctly; otherwise you might be led on a wild goose chase.
