How to activate rear amp on 2000 Yukon-XL?

I just installed a Jensen VM9512 head unit in my 2000 Yukon-XL. I wired a standard GMC aftermarket harness. The front speakers work fine. The back speakers... nothing.:confused2::confused2::confused2:

During my rewiring I pulled the back control console that has a power switch for the rear speakers. I plan to put it back to test, but I wonder is that panel wired to the amp or to the factory head unit.

Any suggestions welcome.

Thanks,

ga2500ev