I just recently bought a 1969 thunderbird. It runs and drives. But there is one weird thing about it. When I put her in drive exhaust leaks into the cabin of the car. Not a little but absolutely fills the car. I can change the air conditioning dial and it changes where the exhaust flows in from. Usually, i have it on off but that's when it flows in through all the vents. When i leave it on De-Ice or De-Fog it just comes in the lower vents. And when I have her in Park this doesn't happen. Can anyone give me an idea of where the leak is coming from or if it even is an exhaust leak?