The yellow one goes straight to the battery side of the starter solenoid.
The red one travels into the dash abiss where it is solder connected to a black with yellow wire which goes to the fuse box in one direction and the other direction takes it to the ignition switch.
I can't explain how the Ammeter works, but it needs to have two positive feeds from two different places for comparison. So, one (the yellow one) comes from the battery side of the starter solenoid and the other comes from the hot side of the fuse box (indirectly).
The attached pics should help with my explaination. The "Violet" wire from the ammeter was actually red on my 67!
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