Wiring Up...

Just run wires from the back of the HU or find where they enter the door and cut them before they go in - just feed them to the back then, jobs a good un! However, I would add a small amp to the preout on your HU (if it has one) and then run that to the back and have an extra pair of speakers rather than disabling the ones in the doors, plus they can be more powerful then rather than a HU standard 50w - on a rear shelf you can get a good kick from a 250w pair or something. More speakers = fuller sound, just upgrade the door ones aswell