I had a coworker who said that in his day teenagers would sneak booze into the drive-in. It became such a problem that the vehicles were searched for booze before being let in. Eventually they got good enough at hiding it, that it was never found. The trick was to keep it in the windshield washer, and run a hose into the cab. I asked if it tasted funny, but he said that running vinegar through it a couple of times first would clean it out.