i tried to fit some 6" speakers in the front and rear doors, and i couldn't get them in without messing with the metal. so, i went down to a 5.25", built a plate to block the rest of the hole and they sound great.
if you want the easiest, most brainless install, go with 5x7/6x8 speakers. generally, these sizes go hand-in-hand. the only discernable differences will be with hole patterns, but 99% of them will fit most fords. including the taurus and ranger.
