According to the service manual for my 380SE (actually, the manual covers various years of the 116 engine from 1981), which I was just browsing at lunchtime (planning a coolant change before winter storage) you need to set the climate control to DEF position (maximum 'on') during the coolant change process.
Also, and I haven't yet verified this by actually looking under the hood, there may be a bleed screw of some sort right above the water pump that is supposed to be opened during refill to allow air to escape....Anyone who is familiar with the 116 engine cooling system can verify this?
Added: I checked under the hood at supper, and there is a hose, not a valve/bleed screw, leading into a T fitting in the hose from the upper radiator tank to the expansion tank, which, according to my manual, replaced this bleed valve in 1981 on the 116 engine. Maybe someone else can offer a method to bleed the airlock from your heater exchanger, where I'm thinking it is...