There is no flow from the main junction (fat dot) to the ignition switch, so the voltage at both should be the same.
There is also no flow between the main junction and the alternator output stud. Therefore voltage in that wire will be the same voltage as the ignition switch connection and the main junction.
There is current flowing through the ammeter to the lights. This current is represented by orange arrows.
Headlights draw roughly 9 amps, and the parking lights draw roughly 3 amps. So the ammeter shows 12 amps going from the battery to the main junction.
It most likely the main splice is solid and for the moment we will assume there ia no resistance to the headlight switch. So the fuse box and headlight feeds are marked -2 Volts as well.
View attachment 1715234773
The 2 volts must be getting lost between the Battery and Ignition switch where the current is flowing. So really, that's somewhere between the Battery and the main junction.
It could be at any of the connections or wires along the way. So check for drop at each of these. Begin at the battery and work from the relay stud (easiest to access) toward the headlight switch (hardest to access). As soon as you find the 2 volt drop, or two 1 volt drops, you've got the culprit(s).
*this drawing doesn't have little red boxes at the ammeter connections, but voltage can be checked there too, and will have to be if the drops don't show up checking the easier to access locations.
I'm not saying the headlight connections are good, but all the information you've posted earlier suggests there is a bad connection or wire between the ignition switch an the battery. Using the headlights shows that at least some of the loss is on the shared route from the battery feed. The headlights should draw more current than the ignition which makes it easier to find voltage drops.
