I would like to know if the following is possible.
I would like to install a window unit air conditioner into a garage which has no windows and brick veneer. The unit would be mounted into a closet wall and I would vent the rear hot air of the a/c into the attic by adapting a duct board box onto it. The unit would sit up near the ceiling so the air would not have to travel very far to be vented out of the room. I would also install a drip pan underneath it which would funnel condensation into a hose and out of the house. Beside not being sure if this is practical, where would the unit draw its fresh air from? Does this come from the rear which in this case would be limited by the closet space? Is it possible to built another duct board box and have this air channeled in from the rest of the garage or from outside (there is an old clothes dryer hole in the wall) the house if need be? We tried a 12000 BTU portable a/c unit from Home Depot and it simply couldn't cool this ~400 sq. ft. room. Want to avoid portables anyway due to their low efficiency and higher costs. Please advice if my idea has any potential.

It won't work. The window unit gets its fresh air from the sides of the unit near the back of the unit. You will spend a lot of money and time trying to do it that way just to find out it won't work. The head pressure will get to high to keep it from cooling and destroying the compressor soon after.

T-TOP, is 100% correct, also you really don't want to add extra hot air to the attic. Is there any way you can just cut a hole in an outside wall? How about installing a small split system? You may be able to find one used somewhere.

Agreed with T-Top & hvacservicetech 07. Also there are those mini ductless systems that work great for small areas