Pick up some baffles and you may need to make spacers to add clearance too. The stock bracket isn't that deep. There's some how to's I've found via google on the subject.
probably cause the magnet in the speaker was to big... you needed baffles to make them fit and so you could still roll window up/down... it an extremely tight fit! I know for a fact on Alpine type R speakers(or components) you need baffles.
I put 5.25 up front and 6.5 in the back. I got the Polk dxi 525's and dxi 650s's. Go to crutchfield.com, put your car in, and they give you everything you need to know and tells you what fits.
I just installed speakers in my 4 door LS, stock fronts are 5.25, however i fitted a 6.5 by removing the factory spacer and making one out of mdf. Dont get a magnet any deeper than 2.25"