the trunks in these beemers are sealed... that rear deck is almost certainly SOLID steel and you'll destroy the car cutting it apart and weakening it. You're sure to get rattles and BMW designed the trunk to be that way for fire protection and to ensure structural rigidity.

I had a buddy cut the deck to install 2 free air 12's in a 7 series and it ruined the car

Trust me...

 

So what do you suggest then , because someone already told me about this firewall dont know what to do with it ???

 

don't do anything!!!! Put 5.25" in the rear stock speaker location and move on to your front stage!!!

 

OK which 5.25 inch's do you recommend , which brand ?
Something nice and loud which i can amplify ?

 

something efficient... YOU DON'T WANT LOUD FROM THESE SPEAKERS... don't waste anything more than $100 on these. Turn the fade right down on them. I wouldn't even bother with an amp unless you want to feel like you're at a concert and your back is to the stage...

 

Well i'll be putting 2 JL or some loud *** subs and i would want sound from the speakers to so i could hear the song rather then having only bass

 

I'm done with this... you haven't heard a damn thing I've said... have fun with your "top of the line" jl system and a$$ sound stage. It'll be the most expensive sounding system at the comps