There should be a fusible link near the starter in that bundle. It will be a 10 ga wire attached to the same post as the battery connects to. Mine was blue. About 12" away from the connection, there will be a bump in the wire. Will look like a cap from a sharpie or a bullet covered in plastic. Something like that. Try to probe for voltage after that link. If you don't have voltage, the link is blown. Auto parts stores have them. No big deal. But, your next move will be to put your volt meter in the ohms mode or continuity mode (beep) if equipped. Probe from the same place on the wire to the frame. If you have continuity here (or a beep), the main feed wire is shorted to ground. That will blow the fusible link. You need to figure out why. That wire eventually goes to the back of the ammeter. If you have had the dash apart recently, is there anything that those posts could be touching? There's some serious wattage connected there, and the fusible link is what keeps the whole harness protected if you short that wire.
However, if you have voltage at the ammeter (your red probe to the posts, the black to a chassis ground), then you'll have to move passed the gauge and into the fuse box. You'll probably have to drop the box so you can probe around. At this point, its smart to disco the ground wire at the battery and proceed carefully. If there is something loose and shorting enough to blow the fusible link, you don't want to find it with your face 6" away!
My guess is its something between the starter and the ammeter. You do have voltage at the starter, right? Otherwise, check both battery cables. Its got to be something simple if the whole truck is dead.
And by all means, check the big bulkhead connector that goes into the cab from the engine compartment. They always come loose.
