Aftermarket Audio Head Unit Illumination Wire

Hello all. I'm in the process of installing a Sony XAV AX 3000 touch screen head unit. My question is about the orange/ white striped "illumination wire". Does anybody know what specific wire in the vehicles factory harness this aftermarket orange and white striped wire connects to? After my research, this aftermarket orange/white Ilumination wire is only to dim the aftermarket headunits brightness when tapped off the (parking light/headlight power signal). Is/was there an on/off parking light signal behind the factory Delco head unit? I'm guessing there is but I'm assuming a factory dimmer wire is not going to be compatible with the design of Sony's "Illumination wire" (this may be true for other brands). I'm also guessing my Sony's Illumination orange wire is designed to be tapped into the parking light signal and not the dimmer. I guess my question is what is everyone doing with this illumination wire. Is the parking light "on signal" location difficult to get too? I have my lower dash off. Anybody know which wire it is? Is it in the FSM? I just want to be clear on dimmer or illumination dim. Illumination dim meaning the head unit dims a shade when the parking lights are switched on providing non distractive night driving. I have posted the aftermarket wiring diagram/instructions. Thanks