92 sedan heater problems, blowing threw defroster only

Guys I need help with my nephews 91 deville. It seems that the heat is only blowing threw the the passengers side defroster. He says its not showing any codes and tthe ECC is acting normal. Any Ideas what to look for before I tear into it? When he says its not throwing codes meaning he never checked and the ecc is reading normal read outs with operation.

it doesnt blow anywhere else, or just wont blow across the whole windshield?

just blows passgenger side defrost duct. NO floor or dash.

I'm not sure about any codes, but i am encountering fairly similar symptoms. Only difference is that I can get air to blow out both defrost ducts. I can't get air to come out of the floor and dash ducts.

Hi Guys,
I have a similar problem with my 91 deville, the passenger side defroster blows hot, while the driver side barely blows at all and what it does blow is cold,(not good in the winter)It will change from defroster to the floor in ECON mode but the drivers side foot duct always blows cold and the passenger side is hot. Also It seems to not regulate the blower speed it is either very low or high.
Plus just the other morning the controls had a mind of their own, apparently it was able to turn on/off the rear defroster and switch it's self form DEF to AUTO to ECON to OFF to back ON again.
I also have to have it set to 90 in order to have any warm at all. I was wondering if the cabin thermostat could be malfunctioning. Where would it be located?
I also get no codes.

check you vaccuum hoses under the hook on the firewall..if all is good its most likely the a/c programmer bad

Does anyone know of a site where I could get electrical schematics for such things a the HVAC control circuit board etc. Electronics is my thing if I can figure out how to fix it I could help others as well.