A lot of the answers are good here but I like the clip ammeter idea where you place it around the negative battery cable to see if there is a huge draw. I worked for Advance Auto Parts for a few years and back in 2012, someone brought in a 1980 Cadillac and I checked it out and he was drawing 23 amps with everything off. I’d say there is a short somewhere, electrical problems can be tough. I once had a 1994 Ford Explorer where mu taillights didn’t work and the whole system needed rewired because they could not find where the short was. I was $800 poorer.
Was there a penny in the fuse box?
Instead of spending $800 to rewire, I would have wired in a SPST cutoff switch to the battery.