Mine does Something similar, on Full blast, the air alway blows partly out of the defrost instead of vents (very weak at vents). Actually, it seems Almost like the blend door actuator for the defrost is stuck in a halfway position. Can't seem to locate the actuator that controls it, is it a vacuum control? Does 1 vacuum unit control all of the blend doors?
Also, I had the passenger side vent blow hot with the A/C on a few times the other day, after turning off/on and cycling the vents, etc it seemed to fix it but I am wondering if the head unit (temp/vent/heat/AC selector with LCD) might be going bad? Do these "go bad" and/or need replacing often?
Anyone else ever encounter these issues and have a solution? Aggrevating Auto Climate B.S....wish I had the manual (simpler) version.