supercuda covered it pretty well.

The sense line (blue wire) goes from the battery, through the bulkhead (twice), through the ignition switch plug (twice), through the ignition switch itself, and maybe another connection or two along the way. Any or all of those can be less than ideal.

My solution was to mount a relay under the hood, have the old blue wire trigger it, and run a new wire through the relay to the alternator/ignition subsystem.

No more overcharging for me.

Edit: if you do find a large voltage drop, you can temporarily install a jumper from the battery + to the blue wire at the alternator to bypass all of the voltage drops. That should immediately bring your charging voltage back down. Note that it will no longer turn off with the key while this jumper is in place

Well if you do that, it will be able to test if that is the issue.
But it isn't a fix, as your car will not shut off.

It seems like you need to route a new blue wire to the alt from the relay directly.

but I think we might back up a little.

so the blue wire coming out of the bulkhead connector is supposed to be 12 volts.

This line supplies 3 things.
The VR- sensing line.
Alt- Other field line
Ballast resistor- 12 volts during running that the ballast turns into 8 volts or so.

How you wired the relay will help determine where the drop is.

You should have cut the blue wire coming out of the bulkhead.
Wired that to trigger the relay and the output of the relay should have gone to the other side/ the end of the blue wire you cut off.
If that makes sense.

If you did that and you still have that big a drop to the alt field wire, then as I said, I would just run a new field wire directly from the relay to the alt.

You can hack out the old one from your harness later to see where the issue is. It could be as simple as the connector is going bad. Or you might have some corrosion inside the line somewhere.

Since it is spliced in pretty far up, do not just leave it hanging or you will catch a dead short somewhere. unless you can find the splice in from the factory and remove that line.

That line splices in near the VR where the sensing line goes into the VR.

So just like the wiring diragram I showed.
on the power out of the relay, you can tie all 3 lines together.
1 wire to the vr sensing line. 1 wire to A field of the ALT.
and 1 wire to the supply side of the ballast.

So just like the wiring diragram I showed.
on the power out of the relay, you can tie all 3 lines together.
1 wire to the vr sensing line. 1 wire to A field of the ALT.
and 1 wire to the supply side of the ballast.