Actually the rear speakers are wired separately ... but they use the same input feed.. they split the line between the Head Unit and amp somewhere in the harness. The amp diagram shows separate feeds for left/right front left/right rear. You just have to cut them at the amp input end and splice in a new lead back to the head unit. If you know what model Head unit you have you should be able to search the internet for specs to determine 2v or 4v output.

Sorry but I don't have anything handy on the ACC line. Maybe someone will jump in that has it at hand. Did you check the threads in my signature links for the CPS and RCM on the shutdown issue?