fumes inside car
 

FNG here. Any help greatly appreciated.

I have an '85 300sd with around 248k on the ticker and it has just started leaking a lot of exhaust through the vents. It is enough to blacken the windows and leave residue on every thing. If the outside air vents are turned off(with O button) it is ok. I think it may be leaking into trunk as well.
The rear portion of exhaust, behind resonators at inlet to muffler, fell off due to rust through and a bad hill in driveway. This was several months back, but I've been ignoring it to try to put off until I can plumb on a larger pipe frt to back. Other than this it has been running fairly well.
It is my daily driver, 67 miles round trip +, and I work seven days a week. I'm coming up on a four day weekend so any hints of what to look for or parts to source ahead would help a lot.

Thank you for your time,
Dan

To add some clarification to FI's statement: not only is the flex tube the weak point, it is located right in front of an air inlet for the interior.

You are dead on. I checked it last night and it is the culprit.
Thanks:)
Any suggestions for a fix without replacing full exhaust?

Thanks for the help. Do you have any suggestions for parts sourcing? So far I've done several parts searches and have seen a wide range in price and actual availability. I'm looking now for an oil pan for my td and have been quoted everything from $235 to $465. I've also waited three weeks for parts just to be told they were on back order then be offered used parts.