1988 Cadillac El Dorado

If you have not been there, check the evaporator core. This can be aided by adding tracer dye, but the refrigerant oil can give a sign due to it's filtration dirt out of the passing air. If you know where the blower control module is situated, you can remove that and it gives you almost complete inspection access to the evaporator. If you don't know where it is, it sits about middle of the firewall in the HVAC case, about betewwn the blower motor, and where the inlet and outlet lines of the evaporator exit the case. If there is no tracer dye in system, or no means of detecting it, look for a buildup(possibly oily) of dirt and fuzz on the lower portion of the core. Some(debris) is normal due to presence of condensation, but if oil has been present it should contrast.