Yes the grill on the dash under the clock is empty unless you have the JBL system. There is space for you to put your own speaker. That harness doesn't look familiar. All you need is a Wiring harness for integrating a After market Stereo to the trucks wiring. You can get them locally at most all Circuit City's and Best Buy install shops for around $15 or online for about $5 Looks like this.
The harness will allow you to use the Factory Plugs and provide Power, Ground, Illumination, Switched power and Speaker wires for the truck.
Where ever you mount the amp you will run the speaker wires from the amp to behind the HU to connect to the purchased harness and plug into the factory plug to feed the speakers you will need to run a Direct set of wires to feed the center channel that you put in the dash.
Hope this helps.