The drain hose is connected to a nipple underneath the battery tray.
Dead cell in the battery requiring it be replaced or something is on pulling power from the battery. Some light or relay.
There is probably a dead short in the system.
Possible the battery is just too old, or it is damaged.
Often it's an alternator diode. Check the battery first. When batteries start to go bad they can often start to discharge over a couple hours. Take it to an auto parts retailer and have them test the battery and alternator for you. I had the same problem with my '92 Windom (ES 300). It was a faulty trunk mounted CD changer that was draining the battery.
When my 1999 did it, I needed to get a new battery.
does a 1991 thunderbird torque converter have a drain plug
Look behind the battery cover for a drain tube connected to a nub on the frame.......drain the oil and change the filter (pre-loaded with fresh oil) and you are done.....there is no drain plug persay.....hope this helps you out......Murph
Check the battery connections.......this happened to me. Usually the positive post (side mount) is loosened.
The battery, cables and starter may be bad.
Charles Drain - politician - died in 1991.