2000 Yukon Rear HVAC Control

It works but doesn't work in auto mode. If I want heat I have to turn all the way to the right. If I want cool I have to turn all the way to the left. I have to use a manual fan setting or it just goes to full regardless of temperature.

I have had the two tees replaced with the corrected ones for better heat.

Any ideas?

the rear does not have heat you can only control the rear fan speed from the rear you can turn it on an off from the front,