Mark,
The lower left right slider may move smoothly but may not completely close the valve in the heater box. There is a ball joint (similar to the ends of the linkages in the engine compartment) that is adjustable like the engine linkages. Ensure that the linkage engages such that the valve in the heater box is 100% closed. This is done by removing the inspection plug, separating the linkage and determining if the valve will move past where the linkage would have stopped. If so, adjust the linkage (consider replacing the linkage-it is probably rusted).
Additional foil backed insulation under the transmission tunnel carpet and the floor board carpet helps keep the heat down.
Best,
Tom