Flush radiator . . . which hose?

I usually skip the T fitting and just shove a tapered nozzle on the end of a garden hose up both heater hoses and run coolant both directions as needed. The line to the the heater valve (which should be open) is the inlet, return line below. That way I can back flush the heater core alone, then back flush the entire engine, leave the radiator cap open to let the excess out there. If you want to install the T fitting, consider getting some copper or brass fittings and make up an all metal T, the plastic T's can get brittle over time and are prone to breaking.