I can't be more explicit.
Plan B. You don't need to find the broken connection.
Just make one.
Can you make a connection from the secondary Tap to the +/- regulator pair "Common" or "neutral" connection ?
This will be a success. Using the same wire gauge or heavier, in a reasonable short path or.... if shorter... connect to same chassis ground.
I added the yellow lines for clarity, sorry to confuse you.
The reason for careful grounds or separate grounds is to avoid creating a few millivolts of ripple on this ground connection near an audio signal ground . Then you get a buzz on your audio.
This neutral ground wire will carry 10x the average current or more as current pulses the capacitors to keep them with l,ow ripple. Any ohmic loss or resistance in the wire or chassis or connection, will create an audible buzz. Basic Ohm's Law.. V=I*R
A thought for Engineers...
As usual, schematics are also called "logic diagrams" and ground connections are often the most "illogical" and go the furthest point on the drawing, or worse, aren't shown and assume you should know. It is even more critical to show proper connections when separating grounds for power, analog and digital to prevent cross-talk by conduction loss.
If you understand, no explanation necessary, if not, no explanation possible. No Worries.