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It sound like the heat and AC dash switch control unit has developed a short and effectively fused some contacts together.
The danger is that electrical shorts have the possibility of starting a fire.
The electrical circuit can be disconnected by removing the heater and AC fuses from the fuse box.
there is a valve in one of your heater core lines in the engine bay that controls the flow of the hot engine coolant to the heater core and is controlled by the hot/cold selector on your dash by a cable. have someone watch the valve while you move the selector back and forth from hot to cold and make sure it's moving the arm on the valve.
oil from the pcv valve going into your intake manifold will cause a gasket to swell. but it should not be to the point where it's noticeable. you should probably replace the intake manifold gasket at this point.
Look for two water lines about 1 in in diameter running back toward the condensor unit in the firewall. On one of those lines will be a control. It is sometimes vacum controlled. When they are vacum conrolled they have a tendency to get stuck or you may have lost vacum to the unit. You can use a vacum tester and short piece of hose to test this to see if you have lost vacum or if it is stuck closed. Connect the test hose to the control on teh hose and apply vacum to the unit. If the valve opens you will get heat in the vehicle. If it does not open then it is stuck in the closed position and will need to be replaced in most cases.
First thing I would do is check your fuses. If all of your fuses are intact, I would make the assumption that it must be the control assembly itself (your actual dash control unit) since to my knowledge it is the only point where the two exclusive systems (heating and AC) are integrated with each other, aside from sharing the duct and to some extent, the electrical system. Being that neither the heating NOR cooling system will work, it seems the only common denominator.
If you have blown fuse(s), remember that fuses almost never blow for no reason (wierd things DO happen on occasion!), and this is usually just a symptom of a deeper issue, such as a short or fault in your electrical system such as your fans, or other components, drawing too much current. Locked fan rotors will blow fuses, for example. Having to replace the fuse again in short order would indeed indicate the need to troubleshoot further.