I think best place would be the ignition barrel, but it's quite messy: you have to remove the steering wheel to access the barrel, then solder in there, and take a cable to the radio... I'd say it is not a good idea. Other places to get ignition signal are the fuse box, air conditioner or the cigarrette lighter, but none of these would work as expected, at least in my car (lighter is always "live"; air conditioner does not work when the key is in "lock" position; and i have no fuse where voltage drops when removing the key from the barrel). That being said, last of these options, the cigarrette lighter, would be the easiest one.
Other suggestions are welcome!