Sorry guys, this is nothing but the Peter Green mod done on a P-90. Assuming you started with both pickups In-Phase and Not-hum-canceling, this will not give you hum-cancelation. All it will do is make the pickup Out-Of-Phase while still being Not-Hum-canceling. You can never have hum-cancelation between two coils if they are the same current flow/wind.
You need to have either both factors Magnet Polarity and Current Flow (wind) to be the same, or both be opposite for the two pickups to sound In-Phase when combined. If you look at this chart, you'll see what I mean what happens if you only change one of the two factors. (You started in a blue box, and swapping one pickup's magnet moves it to an orange box). Magnet Polarity and Current Flow (wind) are two independent factors. Changing one does not change the other and vice versa.
You will need to desolder the hot and ground leads from the braided shield at the base of the pickup, then separate the ground lead from then shield, then swap the hot and ground leads where they exit the pickup, the solder the braided shield back to the ground lead at that point. Sorry there is no way around this. If you swap the hot and ground without separating the shield wire, it will introduce noise into the signal path. Likewise if you soldered the braided as your hot wire at the volume pot or switch. I speak from 12 years pro guitar electronic experience. One of my earliest mods was swapping magnets and leads on an Antiquity P-90 neck to get hum-canceling with the bridge.
