Sub output in BMW Cars

I am hoping someone who has had dealing with BMW/Benz can help me here.

I have had problem getting solid suboutput installed in the boot of BMW cars into the passenger compartment as these cars are so well built. Lot of times customers do not want to modify the rear deck and there is no place for the frequncies to pass through. There is a solid metal sheet between the rear seat and boot. I have tried some  bandpass amps as well to get higher efficiency and louder output. It helps little bit but not much.

Any solution to get good output from boot to compartment without too much modification.

WTF, is this forum getting overrun by Ausies and Brit's? It's in the boot my good man...... care for a spot of tea.....don't mess with the countermeasures Bond....that boot can hold a pile of Fosters mate....I'm poised to start on a monster sub system in a 91 BMW 325ix in the next couple of weeks. These vehicles do present a bit of a challenge, especially when the customer wants good sound, high output and visible subs.