headache....

[Benz300]

Once again my lovely w124 has developed a situatoin even the dealer has trouble diagnosing. so where else but this forum to seek a solution ! :-D
It started last week or so. basically while driving I started to get a headache that I couldn't describe the root cause of. so i opened up the window a little and it felt better. closed the window after a while and the headache was back. so after a little while I figured since the heat is on let me try to shut that off. and there the headache was gone again.
I've figured this much out. as long as the car's vents are being used with fresh air either on heat or cold settings (without a/c) just on the EC mode, something from the vents compartment travels inside the car causing a headache. And to prove that it's not just me I had some friends of mine sit in the car and they complained about the headache as well.
I can't smell any fuel fumes so I'm ruling out fuel leaks. But any idea as to what else could cause this ? I took the car to the dealer, and they were just dumbfounded.
My research tells me that headache is usually caused by carbon monoxide.. question is where to look for such leaks that are only entering the car when the vents are working.... thanks for the help in advance and I really need some advice since I don't want to drive in such cold without heat...