No air flow at max heat setting

I've got a 2007 4cyl tacoma. When I turn the temperature select dial to max heat (Full Clockwise) the air flow slows way down almost to nothing. The blower motor does not slow down, it seems like a damper is not opening.
I pulled both servo motors off the side heater core box and they both seem to work properly. Also I manually moved the damper levers and the air flow stops as I move the air inlet lever from the cold position to the hot position. Is there linkage inside the box that opens a hot damper as the cold damper shuts?

 

I had the same issue on my truck. There is a TSB for it I think it is the blend door seal comes loose and shuts off the air flow. They have to remove the dash to get to it and replace the door. Good Luck with it.

 

The dash is partially off now looks like more is commimg off to get at the box.
Where can I get the TSB?

 

Here you go:
http://www.tacomaworld.com/forum/2nd-gen-tacomas/54785-hvac-vents-t-sb-0072-09-2-26-09-a.html