I would suggest laying a large plastic garbage bag in the footwell under the heater core and the heater valve so that remaining coolant drips on it and not on the carpet. Also about changing the heater valve, more than often, valves have a long lifespan, but the O-ring don't. They dry up, shrink and leak. Those O-rings are dead cheap. Buy them from BMW at $2 a piece or from a local hardware store for 25¢.