Had a very similiar thing happen with the 91 300e in the middle of the city on a Sunday morning.
It was the heater hose that comes from the engine and goes through the firewall by the fues. It was worn where it attaches to the engine, just above the clamp. I used the MBZ tool kit, took it off, cut it back and reattached it.
I know there is a service recommendation to change all the hoses after I think 4 years.
That little plastic connector failing is weird.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)