The Front Heat / Ac Work's ! ! ! ...When It Wants TOO

to me it sounds like a loose wire may be exposed to metal touching that one point or another and it's definitely an intermittent problem and that's what it sounds like to me

Check if it is smoke or steam, if it is smoke do not start the car, if it is steam disconnect AC and go to a mechanic.

It could be your fuse or relay or blower motor resistor

Possible bad blend door actuator .some gm cars can be relearned to the new climate control head

The rear heat and and air will,only work if the front heat or air is on too. Place the front vent selector on the icon for feet and top vents.
The rear console has its own blower too, check the fuse for it.
good luck

The AC sheds the heat from the passenger compartment through the condenser (which looks like a second radiator), which is in front of the engine's radiator. That “preheats” the air that pulls heat from that radiator.

If the radiator has all it can do to shed the engine's heat using the air that is at ambient temperature, preheating that air will push it over the line into overheating.

Additionally, the AC removes the heat in the passenger compartment through the work that the AC compressor does. The AC compressor gets that power to do that work from the engine. This in turn causes the engine to work harder, creating more heat to get rid of than when the AC is off.

If the coolant is old, replace it.
Ensure the belt is tight enough on the water pump.
On many cars there is an electri fan that comes on with the AC, mounted in front of the condenser. If you have this, ensure it is operating.

Your drain lines are clogged, and need to be blown out with compressed air. What you are smelling is mold.

really need to have it in front of me ,could be a fault in the controls .but it must have a switch to turn AC off

The controls for your heater/vent selection is inside the dash, behind the control switches you use to select the vents the heat/AC will come out.
When you move the selection levers, there's a few things that happen. Vacuum is applied to the doors (which are inside) and they open or they work using cables to move the doors or both. The different doors open, directing air flow to the vents selected.
Remove the controls from the dash (which should just be removing some screws, maybe a few small pieces of dash and pulling it out). The controls will be connected inside so they will not come out very far.
Just check to see if anything is disconnected and try operateing the controls with the heat on to see how they work.
Sometimes they just stick or the doors stick. This system is the way the 1991 models worked but since they are the exact same, it should be this way also.
Look it up in some sort of repair manual for more info. I find with these trucks, they don't heat very well using the front vents anyway. I find using the bottom vents work best for heat unless you need defrost.
The "heater" selection will atomatically use the bottom vents, "bi-level" AC will use the front and bottom vents if I remember right and regular AC will automatically use the front vents. For the trouble in removing these controls (the whole dash would need to be removed for repair) I probably would just live with the way they work if they do work atleast properly (enough heat and AC). If you find that you don't get enough heat or AC when in use, check the cooling system for proper operation and blockages/mineral buildup and check the AC system incase it needs recharging or you have leaks.

It's the blowe motor resistor real common