It's amazing what the engineers dream up when they assume we can't be bothered to turn a switch. Turning the feature off most likely will not solve the problem unless the computer is turning the head lights on at inappropriate times. If the computer is drawing excessive current, it will do that regardless of how the switch is set.
To not drain the battery enough to prevent starting for one day suggests a drain of less than half an amp. That would be less than the typical small glove box bulb draws. I'd pop that bulb out first just to be sure that isn't the culprit. Other than that, you need to perform a drain test at the battery. Unless specified differently by the manufacturer, the industry standard is 35 milliamps, (.035 amps), is the maximum allowable drain. At that rate a good battery will start the engine after sitting for three weeks.
You've also mentioned two car models known for developing generator problems. GM's are MUCH more common than Ford's, but if your generator has an intermittent problem, any testing has to be done while the problem is occurring. That means identifying that there is a problem first. Look for dimming head lights, a heater fan that slows down, or a dropping "Volts" gauge on the dash. If you see anything like that, the first step is to measure the battery voltage while the engine is still running. It must be between 13.75 and 14.75 volts. If it is low, Ford provides a convenient test point on the back of the generator for the next test.
Wednesday, January 9th, 2013 AT 8:27 PM