6x9's in the front doors is no problem. All you have to do is just wire up and place in the new speaker and use some self-tapping screws (makes it alot easier) of about 1 1/2" long and drill in the the door frame and you'll be making new screw holes for that speaker.
I thing one would really inquire to do is put weatherstripping around the basket of the 6x9 because that can stop any/most rattles and also make it seat flush and airtight in the hole. I would also seal up the stock speaker bolt holes since they dont get used.
Remember one thing, if your going to run your mids/highs and run the speaker cables back to the trunk along with running subs as well, your best bet is to high-pass your fronts and rears (like 70-90hz) for a start off point. This will make your front and rears play really what they are meant too (midbass) and then you can play your subs loud and not worry about distortion from the other speakers. You can defintely make your system louder/cleaner with just simple tuning.
Power those fronts!! Head units just dont have enough power to make those Infinitys sing. Trust me.
