My experience with american cars would tell me you are right.

Heater core.

Your earlier post described replacing the radiator and therefore closing
any escape routes for pressure.

Check the manual or maybe mercedes.braingears.com for the online manual.

Heater cores are not too bad. Just some time and lots of removing parts
to get at the thing.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250