i recently purchased a "new" 87 190e 2.6 (158,000) seems to have a strong engine & trans. the interior needs a little tlc but functional. when i bought it i noticed the heater core was bypassed, not a big problem in so. cal. but i thought i'd check w/ you guys on difficulty of changing this, and/or how to test the aux water pump for proper output (maybe the only problem?) i've changed heater cores in other cars but this is all german to me!!

Not much has changed since you have changed heater cores before, my guess anyway!

Get a CD service manual, or Haynes manual, they offer one for the 201 also, and learn how to remove dashboard and heater core!

Have fun!

As mentioned, the real fun in changing a W201 heater core is that you get up close and personal with the entire dash board and center console . . . it has to come off. If it's your first time removing the dash, plan for this to be a weekend exploration, but so long as you're methodic (helps to take notes and have a digital camera to remember stuff) it's actually not that fustrating.