English Speakers? Audio Upgrade Help!

[silver50]

......I feel that unless I can get some good advice on how to do this I will have to give up again!

It always sounds relatively simple but ends up throwing up questions like

1) Where do I pick up the speaker input from (if the rear speakers are full range this may help a lot)

2) Do I just disconnect the basic subwoofers and join up the cable (to make a circuit)

3) Do I have to worry about speaker impedance problems

It must be one of the trickiest cars to do this kind of thing on. I have not struggled so much before!!