Most cars already do this if they were built after a certain date.

You may be able to retro fit a small electronics circuit to do the same for a whole lot less sodding around.

Two thyristors (i think) in line one being switched by the ignition and the second by the lights might work. I could do it in relays i understand relay. Thyrisitors my only experince of them is in diesel electric ships where we use them to control and power the main engines