Bmw325I started it up and thick smoke started coming through vents into cab

Bad heater core possibly, does it have a weird smell to the smoke?

of course check the compressor for malfunction but that would never explain a puff of smoke in the cab what did the puff of smoke smell like just a water vapor electrical, rubber,

You very likely have coolant entering the cylinders of the engine, you need the head pulled off and checked for cracks and if ok the head machined flat and replaced

Thick white smoke is usually caused by coolant leaking into the cylinders. You mY have a bad head gasket.

You might want to check and see if you can find any inline fuses going directly to the control panel. I had the same problem on a different car. Just in case.....there should be fuses under dash and under hood. Maybe a relay is even out.

yes sounds like a leaking heater core

The original GM design has a fair amount of thermal blanket material on the lower surface of the heater case, in the engine compartment to protect the plastic from the exhaust components. Check to see if this material is still in place after you had done the exhaust system upgrade.

depending on how bad, and how long the orginal core leaked, it might just be residue in heater box, and if it got up into the vents, it will take awhile for it to dry out.

antifreeze i think,heater matrix leak ,try putting a rad leak stopper in

You have a coolant leak most likely in the heater core. Try disconnecting your heater hoses and coupling them together, then running the engine to see if the leak goes away. If it does, I would have the heater core replaced.