Mike, can you get the heater panel away from the dash enough to disconnect the slider lever? If so, you can check that the slider actually has full and free movement and is not being fouled by something behind or near it. With the panel out of the way you will also be able to see where the cable attaches to the heater box and do the same check there. The reason I mention this ...some years ago, I had a similar problem with one of my Landcruisers and it turned out that the temp control cable outer was attached to the heater box 'tap device' via a flimsy bracket and that little bracket had failed. It allowed the cable to move within the outer one, but only for about half of its designed length...the heat control is only a tap which is used to control the flow of hot water to the matrix (I'm sure you know all this) and all the slider does is turn the water flow on or off, with a degree of control over that flow.
Really sorry if I'm teaching you to suck eggs.
