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I'm no authority on subject compared to some here, but from my recent wiring problems, it seems that power runs from the battery directly to the starter, and from there back to the fuse block via the engine wiring harness. The engine wiring harness clips together to the forward lighting harness at the fuse block, and there's a 12vdc connection (red wire on my '77) between the two harnesses that hooks the power up to the switches, starter etc.
Sounds to me like either a short or a blown fuse. Backups are on a physically different wiring harness that feeds out of the fuse block, separate to the engine or front wiring harness, so unless you're massively unlucky, it's unlikely that you have problems with both lighting harnesses.
What year is it? There are probably folks on here who can help you out with wiring diagrams and so on.