That's a heater hose bulkhead fitting.

One end of this hose comes from the drivers side of the cylinder head, behind the oil filter housing. This goes to the plastic fitting that passes through the front fire wall then to a hose that leads to the heater core if it follows the convention of my 97 C 280 and 97 SL 320. This does not enter the passenger compartment as these cars have a first firewall that leads to the HVAC duct , master cylinder / wipers.

It sort of looks like a universal plastic fitting was installed and some black sealer used to prevent rub through or a broken fitting was glued back on.

in any event, the parts are not expensive to buy. Have a look at the sponsor of this site, Peach Parts. If they do not sell the parts, they are reasonably priced at a MB dealer.