i would check the charging at the battery first with a volt meter between the positive and negative poles engine stopped (12v ish)
then check with car running (13.5 - 15 volts)
if you see an increase in voltage between engine running and engine stopped the altinator is charging and the lamp is faulty check the bulb and the wire from the dash to the fuse box
if the voltage is the same check the connection of the main red wire from the altinator to the starter and the starter to the battery