Start tracing the power wire backwards until you get power and then repair the damaged/shorted wire.


Congrats. Now finish the tailgate and send it to paint.
Backup switch??
That's the logical first place to look I guess...how do you know if it just needs adjusting or if it's bad?

OK, schematic is not real clear on the location, but its shown in the "cab" section, and shows an arc shaped switch which makes me think its on the column, probably under the dash, close to the starter switch.
And it is a combination park/neutral backup light switch.
I bet the wires plug into it in one piece. The plug might be loose.