battery charging 3 plug wires

My father used these on his truck to charge his batteries when driving down the road. I think he ran something off his alternator, then placed a plug by the trailer wiring harness. He then ran a cord to the plug on the boat and drove down the road and it charged his battery on the trolling motor. He really is an ingenious man. I don't have that harness as the truck was plowed into by a kid some years ago.

If anyone can use these, they are yours for shipping (or a fishing trip). I figured that I can stuff them into on of USPS boxes for pretty cheap.