Normally we can use the wire gages provided in the diagrams.
But with the optional wiring on your car, its not quite what is shown in the diagram. Once we figure that out, then whether the wire you want to work on is 12, 10 or 8 gage should be pretty clear.
Look at the wires going to the ammeter in the diagram
View attachment 1716033738
Here's how the codes work.
First letter and number is the circuit. Next letter is the wire segment. Followed by wire size and insulation color.
Lets pick one and decode it.
At the alternator, the optional output wire is R6E 8 BR
R6 = Alternator output
E = First segment (optional wiring only)
8 = SAE wire gage
BR = Brown