[Gravy]

As an ex-service writer, most common items are under hood light, trunk light, glove box light, or security system. Try removing them one by one to see if it changes. Beyond that it's checking all items that use power one by one. Shops are often charging $100 an hour for electrical diag and it can take DAYS to find it sometimes. So, if it's not those, you can see if anyone else has any ideas. See if there is any clicking from a relay either if you remove the positive/neg terminals from the battery to indicate something being on.

If it's a wire grounding out somewhere than your in for a long night.