Big Heat Problem

We have a 2001 GMC Sonoma that has a major heat problem.
All controls are in off position and windows are open, (we bought a thermometer to read the temp in the cab just like this),,,, highest temp 119F (48.5c) with the windows up and all controls in off position the highest temp we read was 109F (42c).
Now we found out the other day that if we have the control in the defrost setting it stops the heat.

We have taken it to 2 GM dealers and both say its normal, one service tech called the GM plant and they say its normal and there is nothing they can do about it.

Does anyone know of this problem and how did you deal with it??

The HVAC system is designed to bring fresh air into the cab with the controls off.
Unfortunately, the air that comes in from the cowl area is also some of the heated air from the engine compartment.
When you put the controls on defrost, you turn on the HVAC system and the problem stops

Thank you for the info.. Is there not a way to have it fixed or something so that the defrost does not have to be on all the time? We took the vehicle to a GM Dealership and they said to put the controls on cold and the vent on ground... we did that but it still got to 109F. Should this not be considered a manufacture defect or did we just by a crappy truck?
This is the only real problem we have had with it other then the squeeking in the back which still has to be brought in to be fixed again.