The Bose amplifier is in the rear cargo compartment (passenger side for a short wheellbase truck - driver's side for the EXT/XL) behind the side panel. The wiring from the head unit should be shielded and I guess yours is damaged somewhere near the door sill of the rear door. Remove the trim pieces and inspect all the wiring you can see for damage.
My 07 tb does this. Its the rear passenger door. Could it be a bad ground. U can hear the turn signals ...rpm increase and such. Bad ground?? Also when the rear entertainment center is turned on (which only then plays front speakers its all gone)