Over Voltage with New Battery Disconnect Switch

This is caused by voltage drop. The VR MUST SEE battery voltage ACCURATELY. This includes the ground path which is a real possibility. Your VR power path (functional) is from battery +, through the switch, cable, up front, through whatever connects it to factory wiring, through whatever path goes to the ignition switch, through the switch, out the bulkhead connector, and to the VR IGN terminal.

"Rig" a wire long enough to reach to the front of the car. This can be anything from no22 scrap ethernet wire to no14 or larger.

Clip that to battery POS, run up front, and connect to one probe of your meter.

Turn key to "run." Engine stopped. Measure as close as you can get to the VR IGN terminal. This might be the blue field wire at the alternator. Do NOT disconnect anything to measure. You want the reading "under load" with "circuit normal."

THAT READING is the voltage drop in the harness. Let's say your read 1.8V. THAT will be ADDED to the proper 14V or so charging setpoint of your VR, and you will then see 14+ 1.8= 15.8V charging running

Then you'll have to chase through all wiring terminals, switches, etc until you find the drop

GROUND. In your case since you say it becomes normal when you change the wiring, grounding is likely not the problem but you should take steps to make it "right" anyhow:

HOW is your battery grounded? Are you using the body? You need a BIG ground jumper from the engine block to the body, an no1 starter (ring to ring) at least

VR.........remove, scrape around the bolt holes of both body and VR, and remount using star lock washers. VR MUST be grounded to "same as battery" neg post.