Just google "back up alarms" or "reversing vechile alarms" and you will find a a large number of makes and types using audio only or all sorts of cameras, flashing lights and etc.
I was just looking at a unit in Northern tool book it has a alarm and a unit to show distance the alarm can be shut off and has two sencors about a hundred bucks was thinking about getting it and mount on each side of our combine and then turn alarm off when it gets on my nerves. hope this helps you too Bob