I have a 1989 gmc sierra 1500 4x4 pickup and when i put the defroster on it only defrost's the driver side.

1-8-4-3-6-5-7-2

Driver side cylinders, starting at the front of the engine, are 1,3,5,7

Passenger side, starting at the front, are ,2,4,6,8

Hope this helps.

I had this happen on my 2004 also. I have the climate zone control module that allows for adjustment for driver or passenger temps. I had to cycle between hot and cold on the passenger side and switch it between defrost and feet, then power off everything for a long period of time using the power button on the control, this finally got the solenoid to become unstuck. If that doesn't work then you have to get into the heater/air ducts under passenger side, which is difficult, to manually unstick or replace the locked solenoid.

Now check if the vacuum operated doors work when you command various vents. There should be a vacuum line from the intake manifold that goes through the firewall and into a pod-like device on the back of the controls. From there vacuum lines run to the various doors. See if you still have a vacuum signal from any of them.

Check the fuse and wiring

make sure the vacuum supply that comes from the engine through the firewall by the evaporator has vacuum and is not broken or leaking vaccum

the mode door actuator is under the drivers side foot area on the left side of the heater box. it has two 7/16 screws and one connector

loosen the bolts and move fender back a little bit then tighten the bolts when the fender is where you want it.

You have a vacuum leak! Defrost is the default position. You need to find vent vacuum lines on passenger side near firewall under hood or under dash.

If you have climate controls that are "slide type" knobs...likely that the cable that controls the diverter panel in the blower box is broken....

Try checking the connecting hoses to the drivers side defroster vents, i also had the same problem. And i noticed it helped a little if you put in on re-circulate instead of fresh.