Where do I connect dimmer wire from aftermarket head unit harness?

I just bought a new Kenwood stereo, and the wiring harness connected to the HU has an orange/white wire marked "illumination". Im assuming this is to dim the HU when the lights are on, but the Metra harness adapter I bought to connect to the car's wiring has no extra wires to connect to this. Does this mean I have to splice into the car's wiring to get this to work? And if so, which wire would that be? (2009 Chevy Aveo).