Alternatively you could just run a ignition-live wire from the cabin's switch to a fused relay in the engine bay and draw power from the battery directly.
As a side note, those roo whistles I have installed are actually effective, but not in the expected way.
1) Roos hear them and ignore them. One particularly intelligent one decided that as I was coming along it would hop right at us as we passed. We didn't hit it - but it was very close.
2) Wild goats run like crazy from them.
3) Sheep also run like crazy
4) Turtles ignore it. In fact, they just pull their head in and hope their structural integrity can hold up against 4 tonnes doing 110km/h.
5) 2-metre long goanna lizards ignore the sound and if you start swerving to miss them, they turn towards you. I think they like whistling. If you're going bush, don't whistle while you work. Unless you're really hungry, of course.
6) Townspeople have no idea what they are. Some move off the road and others deliberately walk onto it, depending on the colour of those lights you see on posts now and again.
7) Yota drivers are oblivious to them and will even stop in front of you, especially if they're faced with those funny red and white signs that say "POTS" or something like that. Very, very inconsiderate.