Heater core
 

My 850 smells like antifreeze inside it.

How hard is it to change the heater core?

easy enough. the procedure is to remove the driver and passenger sides of the center console, then the heater core slides out to the passenger side. you need to drain the radiator but there will still be coolant in the core so the common trick is to use a large trashbag to collect the coolant .

See Robert's youtube vid for the how to.

Unusually easy, thank goodness.