12v DC proximity detector for use on truck (2-3 feet range)

Hello. Well after the 3rd time getting hit at work, this last time was major damage... of course no one said anything. I suspect that the issue is parking. The parking lot is a crappy design. Anyways, I built a bull bar for the rear bumper (its a truck) but I need some type of proximity device that will cover the sides. Especially the rear quarter panel of the truck bed. I would like a circuit that activates the truck horn when it detects an object within 3 feet. (I wonder if there is anyway to discriminate from people walking by?). Any ideas on a circuit? Thanks!

You can get cheap microwave motion sensors from ebay the kind of thing meant for outdoor security lighting, one of these angled correctly might give you the correct range you want.
But it would also pick up people.
To do exactly as you want you'd need some kind of modded metal detector circuit, I've never seen one for that purpose.
Back in the 90's you could get car alarms that talked & made silly noises, they sometimes had proximity sensors on them, you might be able to get the sensors separately still.