You have to test the alternator while the problem is acting up. They almost always test good when you remove them from the engine and test them on the test bench, and it will likely test good on the engine if that is done when the light is off.
When the light turns on again, measure the voltages on the two smaller wires on the back of the alternator. That has to be done with the engine running. One will have full battery voltage. The other one should have less but not 0 volts. If you find 0 volts on one wire, the brushes are worn. They always start out being intermittent. On '95 and older models that assembly can be replaced without removing the alternator from the engine. I don't know if you can do that on a '99 model.
On some models you have to back-probe the connectors to take those voltage readings. On some there are two small nuts that are the test points. They often get rusty and you have to really scratch a clean spot with your meter probe to get an accurate reading.
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Tuesday, January 8th, 2013 AT 2:17 AM