1999 gmc suburban stereo wiring

I recently change the stereo head unit with an aftermarket 1.5 DIN Pioneer designed to fit this truck. I have not been able to get the rear door speakers to operate. I found a posting here from a few years back for a 1996 Suburban that stated there was an amp for the middle set of speakers. I found it under the dash. The posting also said that the pink wire would activate the amp. So, my question is, activate it how? I see a pink (or light red) wire in the dash harness to the head unit that is not used in the Pioneer. There are also two green wires that are not used. So, how exactly to I get the amp to turn on?

The pink wire is indeed the amp turn on lead.There is no such thing as free installation!

If the power antenna wire on the stereo harness is truly a power antenna turn-on you should know that it will only throw 12volts when using the am/fm aspects of your deck (so if you put in a CD those two speakers will shut off).  If your deck harness has a blue wire with a white stripe, that is the one you want to use because it will keep the amp running whenever the deck is on.  If there is no blue wire with a white stripe, it is possible the solid blue wire will always throw 12volts (since it's a power antenna turn-on it shouldn't, but sometimes they do) but if it doesn't, hook the pink factory wire to the decks red accessory wire and you'll be fine).