If the exhaust system has been worked on recently, and the manifolds were off, I would take a close look at the manifold gaskets. Especially the first gasket on the left side or the last on the right(#% and #4 cyls). Very possible the gasket was installed 180* out which will allow exhaust to fill the engine bay and bleed into the cabin.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.