Good Method to Add Wiring For Add-Ons

This all worked well for a while. Now though, there’s no power at that spade connector I mentioned. The problem seems to have coincided with me welding on the car and forgetting to disconnect the battery first.
Anyhow, said spade connector is in the brake warning light circuit. Been checking continuity, I find it in that circuit. I can’t find the source for that current. Anyone have any ideas? I find continuity under the dash, near where the brake handle was, and in places throughout the harness out to the connector at the brake warning switch. I imagine I could find another source, but I’d like to know why this circuit went dead.
We'ld need to know the connector cavity number or letter, or see its position, and then look in the '74 service manual.