Has anyone tried putting 8" subs in the rear deck? Most 8" subs have a cutout diameter of about 7" so it should work. I know you are probably supposed to use a free-air sub but would a non free-air sub get the job done?
no, a non-free air sub will not work well at all if you just install it where the
6 X 9's are. There may be an issue with weight bearing as well. If you are set on using free air subs, the only company I know that still makes some is Image Dynamics (the ID and ID MAX lines). They don't have an 8" sub though, a 10" is the smallest they have. How about removing the 6 X 9's and putting a sub in a small sealed box, or building one in the spare tire well?
If you are going to mount it on the rear deck, you're going to end up with a lot of rattles you're going to hate in the long run. You're better off with fiberglassing an enclosure or making a sealed box to get the best bass. If you are thinking of installing a sub, make sure to get a bass blocker x-over for the 6x9s so you don't have so much distortion.