It's because when the engine slows down it doesn't make as much power off the alternator. With a stock spec alternator your lights get a little dimmer even at idle, so really that's nothing bad.
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2003 Alero GL2 LA1: pacesetter cat back, Energy Speed WAI, Magnaflow downpipe/cat, tinted tail lights, and polished letters on intake manifold.
1987 Honda Rebel 450
1967 Buick Riviera 430 Wildcat: roadworthy but needs restoration.
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