illumination wires on an aftermarket radio are usually orange.
there are two wires in the factory harness behind the radio that have to do with the lights.
one is grey and is for the dimmer one is brown and it is for illumination. i could have the colors fipped but i think this is correct.
the voltage on the grey wire will go up and down with the dashlight dimmer knob. the illumination wire will just show 12 volts when the parking lights are on and zero when they are off.
there shouldnt be any yellow wires involved with the lighting. yellow on the back of the aftermarket radio should be hooked to constant battery voltage which in the vehicle harness is orange.
here is a link to a wiring chart. it doesnt show the illumination wire in addition to the dimmer wire but i know it exists i did installs professionally for 4 1/2 years.
http://www.installdr.com/Harnesses/GM-Wiring.pdf
hope this helps
