Depending on how far you want to go with this the best way to do it is to run new speaker wires as well as a dedicated power line for the amplifier. Now if you are not planning on upgrading the stereo H/U there are adapter plugs available at Crutchfield.
Not sure how far they go for powering up a amp etc. Look them up online and see if there is a plug adapter for the application. I am sure you will have to bring in a power supply for it but there should be a plug assembly for the speakers that will connect to the amp outputs.
Me I strip the interior down to the sheet metal and start from scratch.
A little food for thought.............gotta be a easier way
I have a Nak CD700II that is going up for sale on ebay but its not a cheap unit.