6x9"s speakers are generally replacements for 8" subs when you don't wont to lose boot space by having subs. So you put them on a LPF and let them give you some space. They can be pointed upwards at the window because the bass will 'bounce' off the windscreen and most will stay in the cabin. Higher frequencies on the other hand are much more mono-directional (take a 6" speaker play music and stand in front of it, now turn it 90 degrees away from you and hear the difference in noise). Basically you won't hear crap from 6" speakers pointed at your rear windscreen over the front door speakers, so I wouldn't even bother. If you really want to install some more 6" speakers, than put them in the rear doors.
-Democracy must be something more than two wolves and a sheep voting on what to have for dinner.
James Bovard, Civil Libertarian (1994)
-Giving money and power to government is like giving whiskey and car keys to teenage boys.
P.J. O'Rourke, Civil Libertarian
-Government's view of the economy could be summed up in a few short Phrases: If it moves, tax it. If it keeps moving, regulate it. And if it stops moving, subsidize it.
Ronald Reagan (1986)