This may sound dumb untill you think about it, but i just recently saw a GTP with it bolted to the bottom of the spare tire hump under the car. It benefitted from the somewhat cooler air back there than at the engine area. It was a thicker core unit with a little baffle to deflect air through it. This was a higher point from the ground than the rear suspension main bean still as well. I just seen it so i don't have anymore info. But just off the top of my head i don't know why this location would be negetive versus the radiator as long as you have adequate air (non lowered or added ground affects/arro packages). If you got really elaborate you could duct air from in front of the rear tires as is done for rear brake venting.
