What would cause the battery light to stay on and effect the heat, ac?

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We had the battery replaced and a new alternator installed. The problem was fixed for a while but in less than 4 months the same problem returned. The battery light on the dash comes on and no other instruments on the dash no work nor does the light you can't even see if your are in reverse or drive. The AC / Heat also do not work. Same issue before a new battery / alternator and 4 months later the same problem returned. 2002 Chevy Silverado 1500HD has about 73,000 miles.