How to upgrade mach 1000

[cmaroshetter]

I have a 2002 gt with the mach 1000. I want to upgrade the mids and highs. I have tried putting in new speakers into the doors but they didn't make a difference because the mach system has them setup to play mids and lows. i have 12" sub and amp in the back but i would like to add another 12" or instead have two 10" but the trunk space is taken up with the amp decks that came manufactured. Any suggestions would help.