On all the 86's the air intake has a duct that goes into the cabin under the drivers foot well. It lets you hear the sound of the intake more. The TRD Sound Changer Tubes let different levels of sound in, Red being twice as loud as stock, green being just a bit louder than stock and blue completely blocking the sound. I have attached a few pics of the engine bay showing the tube.
There is also videos on youtube, just search "TRD Sound changer"