it sounds like the relay under the hood next to the fuse panel is stuck on replace it the owners manual will tell you witch one
could be alternator or charging circuit or could be anything got to do with the battery when its draining out loose connection terminal
Check the dome light. These cars are notorous for having a bad door switch.
You will destroy the voltage regulator if you unplug the battery while it is running, then you will have to replace the alternator anyways.
There is a DRL relay in the box under the Hood. You should note that if you are in Canada, the Daytime Running Lights are MANDATORY on all vehicles sold in Canada since 1990, and disabling them may cost you a LOT of money in fines, as it is against Federal law, and the Highway Traffic Acts of most Provinces.
According to the 1999 Ford Taurus Owner Guide : # 30 is a 15 amp fuse for the high beams , daytime running lamps , and instrument cluster
Ford Taurus
How many quarts of transmission fluid does a 2004 Taurus 3.0 OHV require when draining transmission and replacing filter
If you're absolutely certain that the battery is ok (have it tested) and that nothing is being left on and that the key is turning off properly, it's time to have the alternator tested. Sometimes an internal diode can fail which will end up draining the battery. But don't just replace the alternator. Take the vehicle to your local auto parts retailer and have them test it for you IN CIRCUIT! Meaning that they'll test it with the vehicle running. They should be able to determine if the alternator diode has shorted.
BCI group size ( 59 ) Originally equipped with Motorcraft BXT-59 battery ( 2008 Ford Taurus )
36R
12v
Defective alternator, dead cell in battery, or something is on pulling power from the battery.