You got serious shorting problem my friend, don't hook the harness up to the battery until you check and recheck all connections.
For that much damage to happen there must be a power wire grounded somewhere!
You could do the methodical thing, disconnect everything at the connectors then add the battery to the wiring harness, now add each connector slowly until it smokes again, a bit radical but would probably work.
If you suspect the ignition then leave that disconnected but hook everything else up, then add ignition (carefully looking for smoke) if you get heat in your wires again disconnect ignition and have a damn hard look at that.
I have found most of my wiring faults within the headlight bucket, colours sometimes not quite coded right by PO.
Have fun.