88 Cougar Free Air Sub Install

I would not recommend you do a free air set up. The way free air works is the trunk isolates the sound from the back of the speaker and acts as the enclosure. Now if your trunk (box) leaks you will get sound from the rear as well as the front of the speaker which will result in cancellation like you said. The best way to do it is to completely sonically isolate the trunk from the rest of the car. Now a good way to do this is to take out the back seat and rear deck and fill in any wholes and or crevices with caned foam, put insulation in between the rear deck and the rear deck cover.  A good way to test how isolated your trunk is to get some one with a deep voice. Have them talk in the trunk while your sitting in the back seat. Now if you can barely hear them then your trunk is pretty well isolated and free air should work. But most of the time this is not done correctly I have seen some shops charge $400-500 to set this up and it comes out sounding like crap. If you have no low end bass and its sounds boomy you know the trunk is leaking.