If you have back seats then this wont work. But I dont so I utalised the rear seat footwells. I simply (I say that now) created a sealed box that fitted perefectly in the space including a small part that was raised to get over the bump bit in the middle that the drive shaft thingy runs under. Sorry if I am getting too technical
. Then I cut some holes for my 6x9s and put a small (about 50p size) at the bottom of box for the air to move about. It created quite a nice sound, but there was a lot of air pressure comming out of those small holes so before I carpeted it I cut a sainsburys american cookie size (about as big as your fist) circular hole at either end. Then covered it in carpet. It sounds much better and the bigger holes really help. They are just big enough with the carpet making the effective air flow reduced. I know they are meant to be free air, but they work really well in the box, better than when they were on a stealth shelf. Odviosly they arent a match for a sub, but when you get used to them not being as bassy you get adapted to the lack of punch and you get a more rounded sound.
Of course you will need to amp them up and re-alter your sound settings to take into account your running 6x9s and not subs. This said, I am fitting 2 10" vibe subs soon (got em and amps, just need boxes to suite my subs).
So to sumerise they are good, they are just not sub good.
Paul
ps the back looks sh*t hot with that box and a special leveling box I made to level out the entire back end. I will be leveling it all up over summer as there is a little step up between the leveling box for the back and the speaker box.