If I put it into any heat setting, it blasts cold air (feels colder than outside air, so maybe AC is coming on) out of the center vents but the side vents, floor vents and defrost vent pumps out hot air. Any ideas on what would cause this?

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.