The reason for your light glowing dim is more than likely as i thought, with age the wiring or connectors have probably corroded or just need a clean... you need a good full 12 volts to knock the charge light out, so for example say you have a good feed from your main pos on the alternator but the ignition feed wiring from the alternator to your light has a poor connection it'll put a slight voltage resistance on which is enough to leave the warning light glowing but not interfere with charging. It could mean taking the dash out just to clean the contacts but to be honest if you can live with it its not really worth worrying about, could take a while to trace the cause of the resistance. i.e wiring, connectors, even the dash pod itself where the warning lights fed...