radio playing on all 4 speakers but aux on 2?

so i finally got around to the radio on this jeep. not sure what the guy before me was trying to do but big ol mess under the dash. I ripped everything out and rewired it all to the jeep wiring diagram. works like a champ on the radio. when i plug in my iphone to the aux it only plays through the right front and right rear speakers. I have checked and double checked and all the wiring is correct. any ideas why it would be different whether it is radio or aux?