I'm curious what actually happened here. If he stopped payments for his truck, it would be a repossessor taking the truck, not police. If it's unregistered, it's still legal to use on private property (like a farm truck).
I'm guessing his truck was involved in a crime of some sort, or more likey, there's a lot of important information missing.
"I need to show the officer violated my rights, but I have no idea what my rights might be. I just know that it hurts my feelings and this info I got off of a website that sells bathtub gin said I have rights of some sort."