I just put on one of those flush kits from Prestone that you can buy at the autoparts store.
It flushes the coolant from your heater core hose out the radiator.
Then just put in a replacement hose that you get from the dealer.
No reason to mess with the plugs just b/c a german engineer thought it would be a cool thing to put in.

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2008 S550
2010 GLK350
1997 E420 (retired)
2005 CLK500 (retired)