I'd concentrate on the blue ign1(run) circuit that feeds the charging circuit from the igns sw "ign1" terminal to the bulkhead then out to the alt field/VR blue feed wire, an intermittent open/poor connection. Unless wires have frayed insulation on the outside or strands are fried on the inside from a dead short at one time wiring goes bad at the end connections at the terminals from poor connections. Copper wiring does not deteriorate per se from an appropriate amt of currect simply flowing along it's length so I'd concentarate on the ends EDIT wiring also fails from excessive flexing breaking too many strands but this is a rare event & not the issue here. While running at an idle I'd use jumpers & see which one restores charging to pin down which circuit is open. Batt positive post to reg top terminal/to alt blue wire field terminal. alt is good reg is good reg/alt are grounded & there's continuity between alt/reg on the green wire circuit. That only leaves the blue "sense" wire that feeds switched 12V to the alt/reg