i installed polk 651s today. i wanted a rear mount setup just like stock so i could retain the factory grilles. the mounting adapter that came with the polks was pretty useless for this application -- i could not see how to make it work without designing some custom retaining clip for three our of four mounting points. then i came up with a better idea -- use the old speakers to make a mounting adapter.
basically i cut the back off the old speaker and trimmed the face flat to make the mounting bracket. i used a coarse file to smooth out the new bracket front and back. i reused the plug from the old speaker as shown on the evilution site (great idea). i put the polk-supplied rubber gasket between the new mounting adapter and the speaker back. after trimming the polk-supplied screws some i had an easy to make mount solution that used all four of the stock mounting holes. it worked out pretty well!