back in the day when I still used 6x9's there were plastic brackets you needed to use which would convert the 4x10 opening into a 6x9 opening. I think I still may have a pair in the bottom drawer of my toolbox at home.
they look just like these
http://www.autotoys.com/x/product.php?productid=1770
If you want to mount 4 x 6x9's the easiest way ( from past experience ) would be to remove the seatback and parcel shelf material and work it from both inside the car and thru the trunk to cut away the metal for the 2nd set of speakers then use the brackets above for the 2 outside speakers and I think there is just enough room to mount the other 2 in the middle without the brackets. I cut my knuckles many times and the tension bar for the trunklid always gets in the way !
Even easier was just purchasing good quality 4x10's that fit and a sub in the trunk for some added kick.
they look just like these
http://www.autotoys.com/x/product.php?productid=1770
If you want to mount 4 x 6x9's the easiest way ( from past experience ) would be to remove the seatback and parcel shelf material and work it from both inside the car and thru the trunk to cut away the metal for the 2nd set of speakers then use the brackets above for the 2 outside speakers and I think there is just enough room to mount the other 2 in the middle without the brackets. I cut my knuckles many times and the tension bar for the trunklid always gets in the way !
Even easier was just purchasing good quality 4x10's that fit and a sub in the trunk for some added kick.