Dave is right on the money!
A loose or bad ground can take out the power like you are experiencing. Check the negative cable (black) off the chassis battery and follow it to wherever it terminates to ground.
Make sure the connection points are clean and tight on both ends of that cable. While you are in there do the same with the positive cable if you have access to it on both ends.
There is an easy test you can first do to check for a bad ground.
- Get a set of jumper cables
- Take one end the black cable and connect it firmly to thee negative battery post
- Take the other end of the black table and find a solid and clean piece of chassis metal or a good solid metal point on the engine block
- See if you have power
This will basically create a new / temporary ground and can verify if that is your problem.
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