[VetteVet]

The brown wire you are referring to shares the ignition switch terminal with the 12 gauge accessory power brown wire that runs to the fuse panel.
It is a 24 gauge 10 ohm resistance wire at the key switch and is brown and white or brown and yellow. It runs to the firewall block under the fuse panel and connects with the 16 gauge brown wire that goes to F on the regulator.

If you are getting 12 volts at the key switch accessory terminal, then you may not be making a good connection in the firewall block, with the brown wire in the engine bay side of the firewall block.

You could run a jumper from the battery positive post to the brown wire on the F terminal and see if you get charging voltage at the alternator or the battery. You can't leave it though because you need the 10 ohm resistance to excite alternator fields when it first starts charging. Also you need it to prevent the engine from running after you shut the key off because the alternator will feed back enough voltage to keep the ignition system firing.

It will tell you that your voltage drop is in the firewall connection block. If it charges at 14 .5 volts.