1990 VW Jetta Battery Light 

Check the "L"shaped circuit board mounted on the Tach cluster "the instrument cluster has to come out and apart"

As a note, this alternator was put in earlier this year, and the battery is less than 2 years old.

i got the same problem,WV JETTA 2001 same here just replace alternator 2 months ago, check all the fuses, wires,and nothing wrong, im taking my car to a auto parts store to check battery , 10 months old, WILL UPDATE LATER TODAY IF I FOUND SOMETHING,

Did you solve it? I have the same indicator light turning on. I just changed my battery