you guys helped me get the sub and amp setup in the trunk of my 84 monte and it sounds great.
i had to pull the rear seat from my 86 to do some rust repair and noticed the heavy weather shield "tar" paper between the rear seat and the trunk.
my dual 12 inch speakers are facing forward towards the back seat and pulled up tight to the rear seat bracing.
would i get much better sound if i removed this barrier or cut holes on it in a few spots?

It would probably make little difference. Compliance is very high and mass is very low are the reasons why. However, the paper *might* flap around with bass content, possibly making an audible noise. If this is obtrusive, remove it. The paper is mostly a moisture barrier.

Or, to put that another way that may be more meaningful to you, the paper certainly isn't helping your subs.

This is a picture of the subs in my old T-bird, but it looked a lot like this in the Monte. Except I used a pair of grills to make sure if anyone sat in the back seat the Monte's seat frame didn't get on the subs.

You might want to try turning those subs around and face them inside the trunk instead of forward. low bass tones will without a problem make its way to your ear.

I experimented with the subwoofer aiming side to side, forward thru the seat, and rearward into the trunk lid.

By far the sub performed best aimed at the trunk lid, it had more impact, clean hits, and was louder.

I left the trunk jute/tar pad, the subwoofer had no problem sending waves thru the cabin.

I have a single 12” sealed box I reinforced (1/2” MDF instead of more desirable 3/4”) and lined the inside of the box with Dynamat Xtreme, a bit of poly-fill, and silicone in all internal corners. The Dynamat Xtreme made a huge difference.