72 duster firewall wiring questions ???

"Replacing the feed." Should not be necessary. You can buy the packard 56 terminals to replace bad ones in the bulkhead connector, or just string wire through there if you are not that fussy. I got mine from "clips and fasteners" online, they are something like "Aweco" (spelling?)

The thing is, it depends on "where" you are losing voltage. As I outlined........might be the bulkhead, the switch, or the switch connector. You just have to check them. So even if you replaced all terminals "it might be" right in the switch contacts.

In rare cases the ammeter or the "welded splice" is at fault. See the mad article diagram. The welded splice is up in under the dash in the black ammeter wire. It has failed in a few cases.