The Mercedes HVAC system always pulls in outside air from the vents below the windshield. That intake plenum is not perfectly sealed and leakage from underhood exhaust leaks will get in there. It doesn't matter how well the hood closes, if you have an under-hood exhaust leak, you WILL smell it in the car since the fumes will be getting sucked in by the HVAC system.

It shouldn't be too hard to find the leak. If you can smell it, you should be able to see it too. It may help to do your looking at night with a strong flashlight. You'll very clearly see the escaping gasses and smoke.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]