[coolwhip]

I dug into mine a little further.  I remembered wrong, the vent trap door is accessible from the cab above the passenger knees, not under the cowl.  I can see my problem.  When I push the knob in to shut the vent (to shut off heat) it closes, then bounces back a little - thus letting in heat.  You need to be a contortionist to get to the screw to try to reset the position of the closure connection on the push wire.  My simple solution to completely shut off heat for the summer is to push a small block of hard foam in the hole to force the door shut.  Don't block the fresh air vent hole, which draws air from the cab, with too large piece of foam.  Not a very permanent fix, but it works!