Since it's only your rear speakers could be a bad connection/ground in the sound card or amplifirer/head unit. Gosh...noise could be a million things. Eliminating possibilities one-by-one is probably the best thing to do right now. Especially since your car is still apart. Try the sound card, amp, check wire connections, etc.
Bump means to "bump" a thread to the top of the list of new posts. Some people use "TTT" which means To The Top.
Yes, ACC is usually a reference to the accessory 12v line.