Ok, if you just want your custom light to turn off when your other light does, it may be easier to have a light sensor stuck on the other light?
Looks OK to me, although where you have 4 wires in places (on the left) you can probably have 3. Ground plus the active signal.
The programming should be straighforward, I would play with doing it with switches and LEDs for now just to get your hand in.
How were you planning to read the PWM input?
I read that the arduino can natively read PWM inputs via the "PulseIn()" command, would you be able to confirm? If the arduino cannot natively read PWM, would you be able to supply a schematic of an RC filter that would be applicable?
Something like this:Maybe 4.7K for the resistor and 10uF for the capacitor. Other guys here know more about RC filters than me.The output is then the average voltage from the PWM input (taking into account the duty cycle), which you could read with a simple analogRead.