Main dipped headlights not working
You need to look at the factory manual I referenced earlier and use the terminology in those diagrams. That is the only way we will know what you are talking about.
I will say this.........your LOW readings pretty much everywhere indicates a failure at a major point. Start with the Mad article / diagram. Check primary power and follow it to major checkpoints
The entire reason for the Mad article is that the bulkhead connector / ammeter is subject to failure
OTHER failure points in no particular order, due to heat / vibration /corrosion / combination, "etc"
Any and all bulkhead connector terminals. TAKE IT APART and inspect the terminals
Bad connection at / in the fuse link. These can corrode internally either the wire and or the terminal
Ammeter. Loose connections, internal problems in the ammeter, as well as actual failure in the crimped eyelet ends
RARE BUT HAS HAPPENED. Failure of the factory welded splice. There is only one way to inspect this, and only so many ways to find it.
AGAIN...........that splice is a major connection. The power path is battery......fuse link....bulkhead (RED).........through ammeter........(BLACK)..........and to that SPLICE
The splice branches off and feeds
Headlights ONLY power to the headlight switch
main power feed to the ignition switch
hot buss in the fuse panel
Depending on year / model, one or two other items
HOW TO FIND?
You need to LOAD that circuit, IE turn on the headlights. Check headlight power at B1 at the headlight switch WITH LIGHTS TURNED ON
If that reading is much below "battery" you have a lot of voltage drop. Check at the terminations you can REACH. That is you cannot (presently) access the splice
So check at the ammeter, bulkhead terminal, fuse link. If those points are GOOD and the actual wire at the light switch is low, then you need to check the splice
A "secondary" check is to also check power coming into the fuse panel with a load applied. Turn on what you can.........dome, tail, lights, and check the power at the panel buss. If that's low, this confirms, a voltage drop. That power comes from the very same ammeter / black wire / welded splice
HOW TO INSPECT........only one way.......drop the column.......pull out the cluster, and access the harness. Start at the ammeter black wire, and cut / untape the harness a few inches, following the black wire, until you find the spice, wrapped up in the harness.
It is rare but it DOES happen. I found the first failed one of these I ever found in the early seventies, at NAS Miramar, San Diego. This was a friend's 68 Road Runner. We had the entire dash torn apart with my 69 Plymouth manual on my lap.