Chevrolet: Heating/cooling, gmc suburban, coolant level

Question
I drive a 97 GMC Suburban with front & rear heat and air. I noticed that my heater is blowing cold only. Should I be looking for simnple things like thermostat or plugged heater core, or is it likely more complicated? I assume this vehicle has some type of overly complicated vacuum control system.

Answer
Yes, you need to start by checking the basics, coolant level, stat, etc.
The system has both electrical and vacuum controled temp and mode doors but its not realy that complicated. The temp door is the only one that would give you a cold heater problem. If you have the problem of no heat in the front AND the rear, then I would say that it has to be engine temp related. If the problem is only with the front OR the rear then it is a control problem.
If you need anything else, feel free to ask.