Looks like you have a lot of wiring to sort out. 15.3v is still too high. If there is a bad connection in the brown wire, that can cause the high voltage. The voltage on the brown wire at the regulator should be almost the same as the voltage on the battery.
The easiest way to check that is to put the red meter lead on the battery's positive terminal, and the meter's black lead on the brown wire at the regulator. While running this voltage should not be more than 0.5v. That is how you directly measure the difference.
If the brown wire difference voltage is higher than 1v, then there is a problem in the wiring somewhere. Most often it's dirty fuse holders. Next it may be a dirty ignition switch.
You can also check the ground the same way. Put the meter on the battery negative post and the regulator's black wire. That should be almost exactly 0v.
These tests are done with the motor running, lights on etc.
