First I'd borrow a known good battery & set it in there & connect (1) cable only (either one) then wait till it is dark & touch the other cable to the other post & see if you have no spark which tells you nothing is on (doors shut/everything off of course) & nothing should be on at that point (nothing on = no spark). Not sure how putting the battery in with reversed polarity messed up things inside the car (other than the battery) but that'd be a start