2005 Mercury Marquis COOLANT

i DRIVE ABOUT 1000 MILES WEEKLY,SALESMAN FOR THREE DIFFERENT STATES.I NEED TO FLUSH OUT MY COOLANT SYSTEM.HOW CAN I MAKE SURE THAT ALL THE OLD COOLANT IS OUT OF MY BLOCK AND RAD. IF I REMOVE MY LOWER HOSE ON RAD. AND OPEN COCKVALVE ON BOTTOM OF RAD. CAN I RUN CLEAN WATER THROUGH RAD. TO FLUSH. IS THERE A DRAIN PLUG ON ENGINE BLOCK- NOT EXPANSION PLUGS. OR SHOULD I CUT INTO HEATER CORE HOSE AND FLUSH FROM THERE.