One more thing if this may help:

I have just tried swapping back the two units and everything works fine with the MKII.

I wonder if the MKIV unit is faulty (german seller is positive it was working when removed) or if more simply it is uncorrectly coded to a E46 and not to my E53 . Having said that I suppose it should still see the rest of the functions on the screen (OBC, TV, VENT....)

I am totally puzzled !