Is it possible to put an 8in sub right above the rear strut tower in the oem speaker location? If so would it sound like crap?

The car is a 91 FC

 

An 8" will barely fit once you do some trimming. And yes, it will sound like crap. There's not enough volume in the strut towers for most subs and they are hard to seal up.

 

Ok, I guess ill just stick with the 6.25" size then

 

I thought about doing this myself when a buddy of mine suggested it.

It would be much better to use one of your rear storage bins or glass a custom fit hatch box if you want that "clean install" look. Sounds like you do!

Cardboard templates are your friend whenever you're doing custom fit stuff. It makes things a hell of alot easier.

Boom mat makes speaker baffles that essentially turnes your components into sealed enclosures instead of open air. Look into those to incorporate into your rear towers.