I have a a similar problem on my 1991. But it is compounded by the AC not working right now. In other posts I also saw that one or more of the five diaphragm module in the dash may be bad. My center vents always blow out cold while the left and right will be either hot or cold depending on the temperature setting. A dealer told me that on these cars that the center vents will always blow out cold air regardless of the temperature setting, but this doesn't sound right to me. What's the real story on what temp air should blow out which vents in either winter (heat on) or summer (ac on)?