2003 Buick Rendezvous steam

Does the steam coming out of the vents smell sweet and/or leaving residue on the windshield?
If so, it sound like there is a coolant leak in your dash. The heater core is in there and coolant flows through it. One option is to find the leak and repair/replace parts ( which is usually accompanied with high labor costs) or option two, plug or bypass the heater core so coolant doesn't flow through the heater core any more. Doing this will prevent you from having heat from your air conditioner.

It smells like heated radiator fluid coming through..

Could it be the thermostat

Yes it sounds like coolant
Yes it is possible for leaking coolant vapor/steam to be sucked into the interior of the vehicle with the blower motor running. If the thermostat gasket is leaking coolant then you should be able to smell it. Do not allow the engine to overheat as it can damage the engine. A coolant leak will cause the engine to run hot.

OK..Thank you so much!!

Your welcome
Thanks for the question