Best location for 12v switched?

You need to come off the ignition switch IGN1, cut that, feed the switch end to trigger a relay, feed the relay power from a solid new fused source, or after you've bypassed the ammeter. There are a number of reasons for this. Wire is too small, the bulkhead connector has problems, not enough fuse protection for individual circuits, and the switch itself IS ALREADY over taxed

VOLTAGE DROP in the primary power distro and the switch circuit to the ignition IS THE BIGGEST CAUSE of over charging by the charging system