i had 6's in my gsrs doors.
all i did was remove the factory speaker, remove the plastic speaker shield/holder.
cut a piece of 6mm custom wood the same size as the speaker holder and cut the hole for the speaker, screw it to the custom wood and the wood to the factory screw holes on the sheet metal
put the door card back on and walla tweeter mounted up by the mirror.
cross over i mounted in the door pocket, theres prob room behind the door card or behind the door skin as well