Hooking up Head Unit to Dim

Hi I have a Kenwood Excelon KDC-X395 head unit in my 2001 Toyota Tacoma Pre-Runner SR5 3.4 V6 and I am trying to figure out how to wire in the head unit to dim at night when the lights are on. How would I do this? Would I splice into my dash light brightness knob the litttle twist knob that dim's the dash lights? Any help is appreciated.

 

Isn't the factory radio dimmable?

 

I'm not sure I've got an after market but when I installed my adapters all the wires were accounted for and connected so I was trying to figure out where I needed to splice my dimmer wire coming out of my head unit.

 

There wasn't a dimmer wire on the harness adapter? Usually it's just matching color to color.

 

There was one coming from the new hu but none matching from the adapter or harness.

 

If all else fails, theres a dimmer wire you can tap into from the shifter backlighting (assuming you have an automatic). I believe its green, pop the shifter cover off and you'll see everything comes apart pretty easy, just tap into it and run your wire. Its is controlled by your factory dimmer switch. I'm not exactly positive if it works, but its a dimmable circuit.