Heater problem
2000 Lincoln Navigator V8 Four Wheel Drive Automatic 175000 miles
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My blower motor changes from defrost to vent/floor when using my heater. It most likely happens when the blower is on high. It will be blowing out of the defrost and will not engage when i depress the vent/floor. At some point it will alternate back and forth.

I would appreciate any insight you may have.

Also, sometimes my lights dim when idling and then resume normal when I start to go. Do I need to replace the alternator? The belt seems ok.