[PbFut]

Yes but I believe the heater core is out of the AC air flow path unless you have the lower lever slid to the right off the ac position. The lower lever, using a cable, activates a flapper in the air flow path directing to the heater core or the AC evaporator. I had my evaporator out the other day and was observing the functional process of the system to better understand it's intended engineering and observed this functionality. Seeing this makes better sense on what the levers do. The middle lever is attached to an electric switch in series with the fan switch and goes to the AC compressor. There is a white line on the AC panel between the upper and middle lever. Moving the middle lever into that white line area is where the AC compressor trigger is. Left, the compressor is remains engaged. Right the compressor is off. Middle lever also selects vents. The heater core water valve is activated by the upper lever. It is on the same vacuum circuit as the inside outside flapper actuator and the actuator in the passenger footwell. All 3 actuators move at the same time. It seems that the water to the heater core is only turned off when the upper lever is to the left of the line between inside and outside air. I can not say this 100% correct to original design. 50 year old truck, it is possible someone has mixed up or combined vacuum hoses, but it is the way my system works. (72 GMC)
I have sean comments on some systems cooling so well they have to stop to warm up. Simply sliding the lower lever left with the AC in the outside air position will flow water to the core. Siding the bottom lever right a small amount will introduce a mix of heated air with the AC air and provide a more comfortable output.