I just looked at my 1990 Taurus and it does in fact have the external voltage regulator. It is located rigt beside the battery on the fender. I believe your 1991 would be the same. They are easy to change and fairly inexpensive. The regulator tells the alternator how much charge to put out so the battery stays at the correct level of charge and not overcharged. Your light problem is somewhat typical of a bad regulator.