New to FABO

Other than the basic black/red wire codes for DC the traces don't mean much.
Both colors are used to denote hot wires.
Looking at the wiring diagram didn't seem all that helpful either.
Not sure what diagrams you are looking. The factory diagram identifies all of the circuits, colors, and wire gage. The '67 diagrams are not quite as good as 68 but still pretty good. For '67 the Barracuda diagrams are in the '67 Service supplement 67-25-6.
This should be a direct link to the 67 Barracuda Supplement which has the fastback and rally dash wiring diagram.
http://www.mymopar.com/downloads/servicemanuals/1967_Plymouth_Service_Manual_Supplement.zip
if not, its on this page Service Manuals – MyMopar

There's also pdf of the shop manual diagram by @1969383S in this post.
Make sure you get the '67. There are changes each year.

Diagram wire codes covered here

Off the top of my head, gages are powered directly from I terminal on the key switch.
The instrument lights are powered through the headlight switch, then to the 2 amp fuse in the box, and finally up to the connector on the instrument board.

Two corrections to the factory diagram noted here. The difference mostly being that the wire for the instrument lights goes to the steering column connector first after the fuse box.

One thing in common is the grounding. There was an additional ground for the instrument panel to help insure good grounding.

Loose end shown above connected to the bottom of the dash panel under the screw for the radio support bracket.


and
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