if the light is on when the car is off then u may have some wiring problems..
i'd have the altornator(alt) tested.. if it's still got a genarator(gen) i'd toss it and get an alt..
if u have an alt.. make sure it is internally regulated.. it'll save u alot of headaces in the future..
if u have an externally regulated one then u would be better off getting a whole new system.. that way u can also get rid of that box that is mounted on under you hood on the left sideby the rad..(that is the regulator)
an internally regulated alt is just what it sounds like.. no box.. all internal.
hope that helped a bit.