Where is the valve to flush out the radiator on a 1985 subaru GL wagon and how do you get to it?

The valve you're referring to is the radiator petcock. If yours is equipped with one, it will be 2-3 inches from the bottom of the radiator. It looks like a bolt sticking out of the radiator. Remove this and your coolant will drain away. If not, just remove the lower radiator hose and proceed that way. *Remember when flushing out your cooling system, in order to do a complete flush, you must flush in the opposite direction of internal radiator flow. For a down flow radiator (metal fins run vertically), flush from the bottom up, with the cap in place allowing all the coolant and sludge to exit from the upper radiator hose until clear water comes out(best with radiator removed). In order to flush out your engine block, disconnect the radiator upper and lower hoses and remove thermostat and reinstall thermostat housing. Attach water hose to upper radiator hose and flush engine until clean water flows out of the lower radiator hose.