Audio Systems: 94-95 suburban middle seat speakers, aftermarket stereo, cheap radio

Question
Hi there Ernesto.
 I recently helped my sister in law install an aftermarket stereo into  their 94-95 suburban. We used a middle harness to splice to the aftermarket stereo harness to avoid cutting the stock harness.

 Question is, how do we get the middle seat door speakers to work. Wiring as per the harness directions yielded working front and far rear speakers, but no middle speakers. There are 3 wires that are in the factory harness that don't transfer through to the aftermarket stereo.

So, simple question form,,, how do we get the aftermarket stereo to play in all 6 speakers instead of just the front two and far rear two?

Answer
there is two ways to do this  
easy:
turn on the factory amplifier this is under the front seat, to do that use the blue wire from the radio and power the amplifier

difficult :
run two speaker wires to the doors or cables under the seat
if he aftermarket radio is of a good brand it will play fine

if cheap radio it will burn because of the 2 speaker per channel ,that is the reason why gm uses a small 2 channels amplifier