Low voltage at gauge, no charge?

The headlight switch headlights only power (you probably realize) originally got UNFUSED power from the welded splice. This was likely done to INSURE that you would never lose headlights unless the main fuse link blew. It is important to carefully pick your headlight power source so you aren't suddenly "in the dark."

But if both B1 and B2 are getting power WITH THE SWITCH ON (under load) then I would say you are in the wrong area.

Next thing to do is see if the HL switch is sending power down to the dimmer

B1 is headlights only, B2 is tail/ park and instrument lamps