Unfortunately, i cannot give a grammar lesson on this except to say that "hea" placed at the end of a question means "which." "Makemake ‘oe i ke kinipōpō hea?" Which ball do you want? (Your desire of the ball? which?) "Aia" means there is/are. The closest I can describe is that "Aia i hea" is an idiom that has a definition of its own when combined in that fashion. It means where. And to add to the confusion, I mostly hear "Aia i hea [object]" and not "aia [object] i hea," in which the object splits the interrogative, both meaning the same thing. But to answer your question, re-arranging the prompt should be "Aia i hea ka lua?"
Lua is literally a hole in the ground, or a modern day toilet. Lumi 'au'au is the "bathing room." And the fancy pants term is lumi ho'opau pilikia meaning "the room where your troubles are finished," usually referring to the restroom. Memorize the last one. It is certainly the most fun to say, and the most interesting one to hear.
Aia i hea i ka lumi ho'opau pilikia?