Montero 2002 heater blows cool

Verify both heater hoses at the firewall are hot. If they are it means you have hot coolant circulating through the heater core, and the only possible cause for failure is the blend door malfunctioning. This is where it gets hairy. The blend door is controlled by a servo motor located roughly behind the radio, and is not fun to get to. What you need to do is access this motor, and verify that as you turn the temperature knob, it is in fact trying to adjust. If it does not attempt to adjust, the motor has likely failed (you can take it a step further and use a multimeter to check it is acquiring signal, but a wiring failure is highly unlikely) If it does attempt to move (vibrate, click, feel electric pulse, etc) but does not effectively move the blend door, you will need to take the motor off and verify you can turn the door by hand. If the door turns by hand, your servo has stripped out. If you can NOT turn the door by hand, there is either an obstruction blocking the door from moving or the door itself has broken and jammed. Both servo motor failures and jamming doors are fairly common on this era Mitsubishi. Following is a diagram of where the blend servo motor is (called "1 - Air mix damper" in the diagram)