I'm at a bit of a loss myself... I poked around at my '68 wagon and it's pretty hard indeed to figure out where them wires go. I can say that the main feed comes off the starter, not the battery terminal.

From the starter, it looks like I've got a large wire that disappears on the right (US passenger) side of the firewall.. I think that one eventually finds its way to (or perhaps from) the alternator.

Then there is another large red wire that goes through the firewall on the other (US drivers) side. I'm guessing that one feeds power to stuff. That's the one that *was* likely hooked to your amp meter.. That is, your amp meter was in-line on that wire, and you need to find where the other end of it went. Then run another wire to that spot, and to the starter lug (or battery post). That's my best guess.. perhaps later I'll pull the lower dash panel and try to dig deeper for you.

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-Matt I ♥ my ♂