Not too long ago, i noticed that a small amount of smoke was comming out of my vents in my car. The windows were down, so I didn't notice a lot. Now, when I get on the gas, more smoke comes in through the vents. I have a heater core, but it's all disconnected due to it being cracked. I thought maybe somehow the exhause was comming through the 2 lines into the heater core. Is this possible? Where exactly do my vents get their heat from? Thanks