Depends on what you want the finished product to look like. Under the masonite package tray, there is a metal framework supporting the package tray and back of rear seat. It should have speaker cutouts already. Measure them out. I believe they are 6 X 9, but it's been too long since I bought mine so I don't really remember exactly. Get speakers that fit the existing cutouts. I know Panasonic makes them, for one.
You can either cut out the masonite and mount the speaker to the metal frame and the cover on top of the cutout part of the masonite, or somehow slit or drill holes in the package tray to let the sound through and not use the speaker cover (kinda Mickey Mouse just to keep from having speaker covers on your package tray, esp. when package tray is 10 bucks).
I just plopped my carpeted subwoofers in the back seat (nobody ever sits back there, anyway, and if they do, I unplug and move to trunk - the woofers, not the people... although, now that I think of it....).
Dan
Be a good dad to your kids and phone your mom twice a week...
http://albums.photopoint.com/j/View?u=1589115&a=12097562&p=43884454.jpg