Follow the smaller battery positive wire to the under-hood fuse box and check the connection shown in the drawing. Also be sure the battery cable clamps are clean and tight. Follow the smaller battery negative wire to the body. Be sure that connection is tight and not rusty.
Those are the most common places to find bad connections. There should be a large bolted-in fuse. Check those bolts too. If you still haven't found the cause, you'll need a test light so we can do some testing. Here's a link to an article on how to use one if you need it:
https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester
If you don't have one, you can find one at Harbor Freight Tools or any hardware store. For this type of problem, a cheap one with an incandescent bulb works best.
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Wednesday, January 29th, 2020 AT 1:12 PM