Try this (almost positive its your alternator though)
its probably your alternator going, take a wire and run it from the underhood relay on the side inner fender (get it on the same bolt that your alternator wire runs to) run this wire along the top of the fender, into your car, threw the door seal , get a voltmeter and hook it up to the positive and find any good ground in your car and attach it and go out for a ride, if you want have a friend go with you and have him check when the light comes on to check the reading, on the volt meter, i bet it drops, probably a volt or 2 if it does you then should go get another alternator (i'd go get one anyway from what u say, and also my bad lately experience with buying an alternator just go to autozone, i went to ford paid for it then 3 days later they call me and say they dont make them anymore, then i bought the 200 amp from summit made by powermaster (Special order 2 weeks to make 390.00 later), and i had to modified the hell out of my brackets and made it fit, seemed to work for a day, then i had a battery light on, i never had that light on before so i called powermaster and they said there going to take out the part number that summit has in there old catolog for that 200 amp alternator and the guy at powermaster doesnt recommend it due to mods but i modified my brackets and it still gave me trouble, first night ran at 14.75 then next 2 days 12.?? and kept dropping then i went to autozone and bought theres for 130.00 with new pulley or so in stock with a lifetime warrenty (if it dies best bet is to go to a local repair shop and have it beefed up a little) i'm happy with my el-cheapo alternator that i tried to avoid in the first place now... Alex L..