2013 F150 XLT 4x4 EB. Occasionally won't start. Scenario: I turn the key to "on", dash lights up and beeps normally, turn to "start" and theres a quick click and all power goes dead (no dash lights etc). Jiggle the negative battery wire bundles under the hood and there's a click in the fuse box, lights turn on (and radio, even though it was not on when trying to start). Then may or may not start. If doesn't start, repeat the jiggle another time or two and then she'll start. I've looked over the wire bundles the best I could and also ensured then negative ground to the fender is tight. TIA for your help.

 

 

As gDMJoe said
"clean and tighten the battery connections"
Use a wire battery terminal brush. Even when connections look ok they can be oxidized and make poor electrical connections.

 

You do not have a "short", you have the opposite.

 

So, he has a "long", not a bad thing. Sorry, couldn't resist. But, yes, you are correct.

OP, have you cut open and separated the wires of the harness. Wiggle the wires individually and expect each one very closely.