I have an 85 300TDT and I noticed that the controls for the A/C, heat, etc were all not working or at least nothing was coming on when I pushed the buttons. As it turns out the ignition switch was loose and the power to the climate controls being cut off. This may not have anything to do with your problem but this was causing intermittent disfunction. All I have to do is move the key slightly while the car is still running and I can hear a click...the power is restored. I need to take the ignition switch out and see wha tI can do to improve the connections. Go figure.