I have a1973 Ventura with wiring problems.
Faxon Auto Literature in Riverside California may have a factory shop manual for your Ventura. I've bought from them many times. Good quality and reasonable prices. HTH. -Jim
Agree with Js08016. The wiring on that car is relatively simple. About the most complicated part is the turn signal/brake light setup. Make sure you've got a solid ground at the lights that are failing, and just trace each wire backwards. If you're trying to keep the car factory-authentic it adds a bit of work for you. Tracing electric problems can be time-consuming, no easy fix. Good luck!
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