Assuming that you have taken the head unit out of the dash, you can feed the wires up from the passenger footwell. If you lean into the passenger footwell from the passenger side and put your (left) hand up behind the centre of the dash and feel up towards the back of the head unit you will find there is about a 2inch gap big enought to fit cables through.(heater ducting prevents you being able get your hand right up there)
You then need to locate the speaker ISO block on the back of the head unit and connect your new speaker wires to there. The new wires can then be fed under the passenger side carpet and then to the rear of the car. (I wouldnt attempt to rewire the front door speakers)
I personally wouldnt bother messing around with new wires because ultimately your new speaker wire will still be connected to a short length of the standard wire from the ISO block, unless you intend to connect the new wire straight in to the existing ISO connector (quite tricky,) you are unlikely to experience any difference sound wise. If you havent had the head unit out yet, you will see what I mean when you do. For what its worth ld use/extend the existing speaker wires and leave the head unit alone unless you are going to replace that as well.
stu