I am not aware of any #10 gauge wire in the original harness. The no.12 gauge wire that I think you are referring to is pictured in this diagram. It runs from the alternator to the small soldered junction in the OEM harness and to the battery. It also feeds the cab through the firewall junction.
Here is a smaller version with the junction circled.
The 18 b/w and the other 18 ga. black wire at the battery junction are for the battery gauge in the gauge dashes. The charging light dashes do not have these two wires in the harness.