TL : DR → Headlights dim and blower speed slows (only when set to highest setting) at idle, then brighten / speeds up when under throttle. Changing the alternator didn't fix. For a while now I've noticed that my 02 Denali's headlights and dash will dim ever so slightly at idle, then brighten up when you step on the throttle. Also, when on the highest setting, the AC blower motor will slow a surprising amount at idle, and speed back up when you throttle up as well. I thought this was a sign of a weakening Alternator. Over the Mother's day weekend, the alternator died. I limped home on just the battery, then replaced the alternator and the battery. The truck starts and runs great now, but it didn't solve either of my dimming / blower speed issues. What else would cause those to happen?