Heater issue

[Lucas0125]

I posted before about the heater at the vents blowing with no pressure. Did some research and it turns out that is by design but after doing some checking there us a new issue.

It seems the lower vents will only blow cold air when the fan is on for anything. If I put the fan on high and select mid vents then I get cold on the bottom and heat from the middle. If u select defrost I get cold from the bottom and heat from the top.

Any ideas?