I'd remove the tank (remember to remove the fuel pump relay to avoid the risk of spraying fuel if you power the ign circuit )
then with the ign off disconnect the wires from the regulator until the dash lights go out (+ve,-ve ,dash light and the 2 generator wires if I remember correctly)if it doesn't have any effect then the fault may be elsewhere, it may be as simple as a short in the wiring around the headstock ?
When my replacement regulator went faulty last month (30 amp fuse blowing) I discovered the regulator case was live due to an internal short , with a heat gun I was able to remove the potting compound and see a burnt area between the board and the case maybe due to water getting in ?