>What is the proper position of the Defrost sliders to make sure that it is OFF?
The two lower levers control the heater for the left and right side of the car.
The red triangle denotes less or more head, more heat toward the larger part of the triangle. So to shut the heater off, those two must be moved toward the center.
The upper left lever controls outside air. Again, look at the triangle, blue in this instance. You get more air toward the larger part of the triangle, ie. toward the left. The last ~1" of travel makes the fan come on, it has 3 speeds. So for the fan to be off, move the lever toward the left until you feel a little resistance. When you engage the fan you will feel the lever engage into a notch. There are 3 notches for the 3 fan speeds.
To get no outside air into the car, move it to the right.
BTW, you can also control outside air going directly to the left round vent with the lever next to that vent.
The right upper lever determines where the air comes out - push to the left to defrost and to the right to get air to the floor.
The w108's HVAC system is a bit weird, as the Heat and AC systems are, I believe, separate.
AFAIK you cannot get AC'd air to the defrost vents, or heat to the round side vents. The heater either blows to the windshield, or to the floor.
The AC system is also plumbed into the left vent's fresh air inlet, so if you have the AC on make sure you close the fresh air inlet with the lever next to the left round vent.
My AC blows mostly from the middle vents and very little from the side vents.
It has been converted to r134a by the PO, and it works OK, but not great.
BTW, nice car!