Alternator/generator/regulator problems of the 2003 Ford Thunderbird

A couple of months after the car was purchased the battery light comes on and when it does no other electrical parts will operate. For instance if you have a window down you can't roll it up or the heater won't work. This has been an ongoing problem since the car was purchased. We've had the car to the dealer 3 times and they have replaced the alternator as well as overlaying electrical wires and none of this has fixed the problem. On one occasion they had the car for a month, put approximately 200 miles on it and still couldn't find and fix the problem. Our concern is that when the battery light comes on and nothing else electrical works what if you needed, say, the defroster to come on and it wouldn't it could pose a serious safety problem! the date in the incident box is just an approximation this has been ongoing since we've owned the car. Thank you.