Shouldn't matter too much. Get the proper size, 5/8" I think is right, and put it in one of the heater hoses near the top of it's arc, so that the tee is where the air will go. Tighten gently, but firmly. Check often for leaks. Make sure that it's away from really hot parts, if it's plastic like mine.
The heater core circuit is independent of the thermostat and so will always flow according to the heater lever setting.