2000 Cadillac Catera Heater Core/AC

The car is pretty much all german, an opel. The heater core is behing the center of the dash, requiring dash removal to service. Are you sure it needs a core, just because it has no heat? You have several other possibilities to consider, the only problem a core could have is plugging. Does the engine seem to be getting hot enough, maybe the thermostat is stuck open. You also have a heater control valve that may be non-functional or the vacuum hose is off of it. You could also have an auxiliary water pump that is non functional that will cause reduced heater performance. As far as the mode issue is concerned, it could be a faulty actuator, control panel, or something in that area. There are codes that set when faults occur, and you would not know they exist. It would require a scan tool that is capable of talking to the heating system, to extract these.