You need an adapter bracket. This link should give you some ideas:
Find what fits your car - Crutchfield

Also if you take a picture showing the speaker and the mount we can come up with ideas on the forum. If you rig something please take a picture. That might help the next person.

Oh BTW I have a 2006 and find the rear speakers to be fine. They just need more power than what stock provides. It takes power to reproduce a sound well. Notice I said not loud, just well. That is why you will find these guys with super power amps and play right a comfortable listening level and not blasting.