If you are going to own an old car, and especially one with 42 year old wiring in it, a multi-meter is absolutely necessary. Get a good one when you buy one, and it will always give you accurate readings for many, many years to come. You just test the wire at the 12 volt source to make sure there is 12 volts there, then go to the other end of the wire and see if you have 12 volts there. If not, then the wire is broken somewhere in the middle. Just wire around it and pull out the old one. Oh, the best brand of multi-meter is "Fluke" I have had mine forever.

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Ah, a Fluke 75, brings back memories, best meter I've owened so far (before I put 15,000 VAC through it). Now I have a 77II and hate it. Looking for a new one maybe 179...