Depends on whether you are starting from a 6 speaker system or 10 speaker system. US cars, I believe, were sold with a minimum of 10. My Oz delivered diesel came with only 6 (my fault, presumed that the standard BMW system would be as good as the standard fit in my wife's Golf, wrong!!!). The difference? The 6 speaker system does not have an external amplifier, relies on the amp built into the radio unit in the boot. The 10 speaker system has an external amp which is located below the cd/dvd in the L/h side of the boot.
If you have the 6 system, it would be more difficult, you have to come up with 4 more speakers (2xtweeters in front doors and 2xmid-range in the rear doors), 2 amps and a sub-woofer. The 10 system, would be as the other guys have already said.
Colin.