I had da same problem wen changin da rear speakerz in my rolla. Mine weren't 6x9" dey were 5.5" to take out the rear carpet piece you need to take out the anchorage point which iz a bolt that goez from the top of the parcel shelf and through the other side of the frame. To do this you need a high powered oxygen pump wiv the socket attached as this bolt cannot be taken out easily. Another thing too, it may be illegal to do this, unless you take it to a professional installer. After you take this out, the speakers are mounted in pods just unscrew disconnect rewire your new speakers and mount back in.
In my case I couldnt get the anchorage point out so I took of the speaker covers by carefully ripping them of the rear piece, BUT I'd advise you dont do this unless you are planning on re-skinning the rear shelf or installing pods as the gap that is left around the speaker doeznt look too good. Im at the moment looking at reskinning my rear shelf with vinyl or material or maybe making custom pods that would cover the surface.
The following is a link to a page that has instructions on making pods there are many others but this is the easiest I hav found so far.
http://www.geocities.com/lincandyr/
The instructions are made for front doors but the concept can be easily modified for rear shelf. You may even be able to redesign the pods for 6x9s if u want.
Good luck! msg me
gansta_natedogg@msn.com if u need any more help or advice whether for installation or for products etc...