1998 bravada has six radio speakers; two dash

1998 bravada has six radio speakers; two dash speakers and two in the front doors speakers plus two rear doors speakers. the front doors speakers work good but no sound come from the dash speakers or the rear doors speakers. using the fade and balance botton I can get the sounds to go right or left on the front door speakers but I can not direct the sound to the rear speakers. Oldsmobil installed radio system . Delco electronics , doby sounds.