68 Barracuda Instrument lights not working

If the diagram is to be beleived, these circuits are all spliced to wire M1.
If true, you'll find one wire (M1) at the fuse of your '64
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However if you do, check the connection at the headlight switch's B2 terminal. Manufacturing may have found it more practical to join the wires at the terminal than make a welded splice in the middle of the wire.