I'd be finding the source of the exhaust leak and fixing that first. The fact you can see exhaust smoke coming up the firewall says that even if you fix that grommet, you'll continue to have issues with exhaust intrusion into the cabin. You should only be getting exhaust out the end of the tailpipe....nowhere else!

__________________
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]