GM-GMC: Front vents not blowing, electrical connector, heater core

Question
I have a 99 suburban and when the heat is on it will blow good through the
back vents but.it will blow cold air through the defroster and the front vents
will not blow any air period.

Answer
Depending on the type of HVAC system that you have, auto or manual and assuming that the fan itself is functioning as designed, there are a few simple checks that you can perform.  I suspect that the one of your mode doors is not functioning, not allow air to flow over the heater core and into the dash.  Inspect the mode door actuator located in the front centre under the dash.  It is approximately in line with the ashtray etc but under the dash a bit more. You may need to remove a plastic cover or shroud in order to access it. Ensure that the electrical connector is attached to the actuator and that if the actuator moves that it moves the door inside the duct properly.  If thing appear to be in order there, look around a little bit to the right of that actuator there will be another, perform the same checks to it.
Hope this helps a little.