Is there a way i can remove the metal piece that is behind my back seat? if so how? best way? and would it increase the volume or level of sound in the cab bass from my subs?

Depending on what you're specifically talking about, the metal cross piece behind the rear seat can be an important part of a unibody'* structure... I suggest you leave it.

i say leave it. that would not only be a pain in the butt to get out, but also adds support to the body. what year/ trim level you got? i got a bit of an idea.

its a 91 sse... well ill take any ideas..... i was thinkin sumhow makin ports from my trunk to the cab to let sum bass in

Probably your best bet is to make those ports on the rear deck in the car ( where the rear speakers mount behind the rear headrests). It will allow the air/sound to travel into the car and won't be as hard as trying to get rid of the steel "wall" and less chance of affecting the strength of the unibody. Plus your seats are still there somewhat blocking the sound anyhow, unless you planned to leave the back of the seat off.