How do I troubleshoot electrical issues

First I'll say, I see evidence of a squirrel having been under the hood. (acorn shells/scraps) Truck has a new battery, but is dead, dead.   Now, when I turn the key in the ignition I get nothing, but when I turn the key off it gives a low clicking sound from the back right side seems like under the truck.  I will need to jack it up to access! Nothing looks afoul up top of the engine.  I disconnected the battery to get the clicking/purring noise to stop.
I'm baffled.  I see no damage to the wiring harness behind the battery case!