Could be an alternator negative diode leaking (allowing current through the wrong way).
Looking for a small leak has already been described.
If it lights with this test, yes definately an electrical leak.
If it doesn't light, there still may be a small leak, but the resistance is too high to get the bulb glow.
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If the multi-meter only has miliAmps, it may be worth first checking as a dartfreak suggested.
Higher currents through a meter can damage them. A meter that can handle a few amps will usually have a fuse in them.
