Since the rear speakers are so far away, I'd just cut the harness. If your dead set against cutting the harness, just run a long jumper and hook up the rear channel like your doing the front. Since your wiring the amp by high level input you won't need a remote power wire either, just battery power & ground.
2010 FLTRX , Stage 1, 13" Madstad, Mono tubes, SuperShox, Heritage style bars. Alpine CDE-HD149BT,Soundstream Picasso Nano Series PN4.520 , 5X7 and 6.5 Infinity Kappa speakers
If I have to explain ,you wouldn't under stand anyway.