Q: Smell coming from A/C vents

There is an odor in my car that comes in through the vents of the A/C system. There has been a leaking radiator, but it was replaced. That ‘had’ been a source of one odor, but now there is a 2nd one and it is literally giving me headaches. At times, the smell is rather metallic and like hot exhaust fumes. Yet, it also smells like hot fluid similar to power steering fluid or anti-freeze. The dealer did a pressure-check on the cooling system and reported to me that it is not a heater core leak. I replaced the fresh air filters. Around the filters, I thought there was a gap and leak, but they appeared fine. The smell is only from the driver’s side a/c vents. When the air flow is on low, the smell is more pronounced. I crack the window or open the sunroof. I had also thought that it might be the exhaust back-drafting into the car. The dealer has not helped me one bit, despite the expense! Can you give me suggestions on the cause of this problem and how do I solve it?