I have an 03 GT and i dont why, but my battery light decides to turn on and off when it feels like it. battery is only a couple months old. Alternator is a little under a year old and so is the belt. already checked belt is not loose. It starts up good and still runs great. any one else ever had this problem or knw what could be wrong??

 

The battery light is linked to the output of the alternator. That being the case The alternator may be going out, normally the "new" alternator you get is actually a re-manufactured unit. The other possibility is the battery cables, ground cables, or alternator cables have become loose or corroded. Clean and tighten them and see if the problem is solved if not you'll have to start looking for a failing part.

 

Mine did the same thing. Bad battery terminal on the negative. Then the alternator went 40 miles from the closest auto parts store in the dark. That was a fun night.

 

Loose/bad terminals is usually the cause of that stuff.

 

I'm having the same problem. One week after I got a new battery, my "battery" came on after I left work. I took my car straight to Pep Boys and they couldn't find anything wrong. Both the battery and alternator were working just fine. A few days later, I took it to Goodyear and they told me that the best thing to do is take the car to the dealership because I could have a loose sensor.

 

http://iihs.net/fsm/?dir=361&viewfil...g%20System.pdf

Take it to autozone, get the codes, if any, assuming you don't have a tuner/ODB-II reader already. See above. You can then also start the car and run it, monitoring that voltage sensor to see if it's seeing under/over or simply loosing communication.

A good general video about charging system diagnosis:
[youtube]FTGz0PKIl84[/youtube]