01 f150 164,000 miles. Air will not blow out dash vents. this started 2 weeks ago. was working fine and then would blow out defrost vents when the engine was under load and then switch back to dash vents as load eased on engine. figured it was vacuum problem, maybe loose line or leak. now it only blows out of defrost vents. could this be the control switch and is there any way to check it. All other AC/heat functions work normal.