<<What do each of the 4 heater controls do starting from the top moving down?>>

Simplest lever first, the 2nd controls the amount of heat. But the hot air to the windshield is mixed with some fresh so it's never as hot as cabin air.

The amount of fresh air that comes in is fixed and always "on." The top lever determines what percentage goes up to the windshield or down to the cabin. Of the amount that goes to the cabin, the 3rd lever determines how much of that goes to the foot wells. The amount that goes out the dash vents is controlled by their individual grill levers. So if you shut them off, you get more to the feet -- remember, fixed amount, subject to vehicle speed, coming in.

Finally, of the lower cabin air, the 4th lever siphons some off to the vents over the sliding door. This rear cabin air is never heated.

The A/C vents do not control any fresh or heated air. Conversly, no A/C comes out of any heat & ventilation system ducts.

Engine and vehicle speed affect air temperature and flow. So what is a good setting at highway speeds goes to pot in town.

To shut off all outside air, for example with A/C on: top 2 levers left, bottom 2 right, dash vents closed.

For maximum defrost, all levers right, dash vents closed.

Best cabin heat compromise and start point for personal adjustment, top lever 1/4 from left, 2nd & 3rd l/4 from right, bottom full right.

Hope this helps,

Troy