Most of the L-body cars are starting to exhibit a common problem with the heating and air conditioning modes becoming inoperative or stuck in one mode or the other due to the vacuum control harness coming apart at the plastic splice connector. The heat and A/C hand control located inside the passenger compartment and on the left side of the dash is where you adjust the mode, the fan speed and the temperature of the air discharged.
This control unit uses a series of vacuum lines to control the mode and amount of heat that is produced. If these lines become loose or disconnected the system will often get stuck in one mode or maybe the heat wont adjust. It can also cause running problems if the vacuum leak is big enough because the system pulls it's vacuum source from the engine compartment.
The common problem here is that the vacuum harness that is attached to the back of the control head, has a splice connector in it, about 6-8" from where it plugs into the control unit. That splice connector, over the years, falls to pieces and the lines become disconnected. It is difficult at best to try to guess which lines go to where when this happens. So below is a picture of the connector in good shape and showing the location of the different colored lines. I have discovered this arrangement works on 85, 86 and 87 L-body cars that are A/C equipped from the factory. It may be different for non-AC cars.
To repair this, simply use short pieces of small diameter silicone hose or windshield washer hose and something like a zip-tie on the ends to hold them snugly in place, and splice the lines back together as shown in the pictures below.