There is a coolant control box for the heater under the hood in front of the driver position (on a V-8).
These can clog and fail, in default mode it is supposed to be open to allow heat into the cabin. If not obstructed (like in your case) it will pump hot water.
Check the voltage at the box. Need to remove the clips under the cover if the voltage is there. They are exactly like old GM door handle clips. Might need a mirror to see where to push. Take that cover off and then check into it.
95 E34 530I V2.37
===========
Those who make peaceful revolution impossible will make violent revolution inevitable.
John F. Kennedy