When I turn on the heat in my '06 Aveo, the air coming through the vents smells smoky and a little bit toxic, almost like exhaust. This problem did not exist last winter. Any theories?
The smell could be one of a few things. As long as you have all your heater-A/C fan speeds working fine, we can pretty-much eliminate a burning odor from something electrical.
It's either the heater core in the dash leaking and you smell coolant coming out the vents (check coolant level, fogging windshield, and under the floor mats to see if the rug is wet on the passenger side), or there's an exhaust leak under the hood. Exhaust fumes can be drawn into the heater-A/C system through the fresh air intake opening located at the base of the windshield.
You have to be sure about what you smell: burning rubber, coolant or exhaust.
From my angle it's most likely coolant, but if you're sure it is exhaust -- it's not safe to drive. Have it checked out by a technician ASAP.
Note: Mold/mildew odor coming out the vents is also possible, but again, you have to be sure of the smell.