For some unit like GMC 07-14,Tundra,Titan, GX470, escalade and some other unit which only have sound from one speaker or in the front after installation. Check the setting below

In the console on the top icon of the unit. 

You will get into following page, sometime it's in the amp info setting in the console, or the small gear icon in the console.

The setting FR(front and right), LR (left and right)

Adjust those setting from 0-14 to get the sound for all sepaker.