What kind of amp are you using? By doubling up the rear channel you're changing the load, make sure your amp can handle it, or it will shut off at higher volume, or may not play at all.
Assuming you're using an aftermarket wiring harness (Metra/Schoshe/AI) you can save yourself some speaker wire by running wires from the amp up to the back of the radio. Then hooking up the speaker wires from the amp the car side of the wiring harness (not directly to the radio). Doing that saves a wire run directly to each speaker by using factory wiring. You may still have to run wires from either the 4x10s or from the rear 6.5s as aftermarket wiring harness have provisions for 4 speakers and the truck has 6.
On my Tahoe I have the power and remote line running on the drivers side and the RCA and speaker cables running on the passenger side. And by the way, I have my system powered from the aux 12v post at the fuse box. Im not running anything crazy so it wont be an issue, otherwise Id go straight to the battery.