i had the same problem with the heat. it was the stat for me. and i modded the ac fan to turn on by switch since it never turned on anyway. if it was a head gasket check the oil for coolent in it if there is then that is ur problem. are u seeing any smoke coming from the engine then it might be underneath the injector rail and it can be a oring leak on a pipe going to the rear of the block. is ur carpet wet with coolent on the drivers side. if so then it is ur heater core....it can be a number of things. hope this helps

1990 Montero lwb, 3.0 V6, Custom bull bar, Completely stock on everything else. 186,000 miles
2" inch torsion bar lift. Conferr roof rack with homemade gutter mounts.