Every time Derek has found himself in the back of a police car he could always link it back to Stiles. The first time was when he was accused of murder. The sheriff himself had come to his burnt-out shell of to personally arrest him for the murder of his sister. He had sat in the back of the car, watching as they unburied his sister from the makeshift grave in the back of the house. He'd been so focused on that he barely even notice when the boy who had been with Scott that time in the woods climbed into the front seat of the car.
"I'm not afraid of you." was the first thing he'd said. Derek could tell it was a lie; the stench of fear was thick in the air around him and written in the ridged stance of his body. The exchange was brief and ended by him being pulled from the car abruptly. Derek listened to the exchange between the sheriff and his son (as Derek had then realized) and couldn't help smirk that formed on his lips as he listened to Stiles be caught up in his own lies. They wouldn't last more than a day without him, he'd thought at the time. How wrong he would come to see he was.
The second time Derek found himself in the back of a police car was far different, but was still entirely Stiles' fault. In the back of the police car Stiles had borrowed from his dad, Derek seriously wondered how Stiles had convinced him this was in any way a good idea. Dating the Sheriff's son in secret was bad enough without adding auto theft to the equations, but Derek had already come to terms that if Stiles wanted something bad enough he could convince Derek that even the worst plan sound somewhat do-able when coming from Stiles perfect lips. Especially when it required those lips to be on his own, kissing him hungrily as both tried to remove the other's clothes without actually breaking the embrace.
It should probably frighten him that he was so willing to give Stiles this much power over him, but he trusted Stiles more than he trusted anyone besides maybe Laura. Even after what had happened with Kate, he knew Stiles would never do anything to hurt him. In fact, Stiles had proven over and over again that he was willing to put his own life in danger to save him. So when Stiles had suggested having sex in the back of a police car, how could he have said no to him.
The third time wasn't entirely Stiles' fault, but in a way, yeah it was. It was also Derek's fault too. Sheriff Stilinksy none-to-gently threw him in the back of the car causing his arms to bend painfully against his back and slammed the door. Even through the glass he could hear Stiles trying in vain to reason with his dad. They argue about consulting adults and acceptance and making a scene. Derek is about to say something, anything to calm down the two men arguing quietly outside the car when Stiles said "I'm in love with him dad. Please don't do this." It was enough to shock him into silence from whatever he was going to say. He sat there, handcuffed and half naked and completely shocked. It's mortifing, but also the best moment in his short cursed life. Stiles turns from his dad and meets his eyes through the glass and repeats it. "I'm in love with you." And Derek leans his forehead against the window and says it back knowing Stiles wont be able to hear him, but hoping he will read it on his lips and in his eyes. The Sheriff looks away, his shoulders slumping in defeat, before throwing open the car door. Derek almost falls out, but Stiles is right there, catching him as he always is and before Derek knows what's happening, Stiles' lips are on his. Now they are definitely making a scene but he doesn't care. Stiles loves him. He loves Stiles. They're in love and yeah, It's really fucking great.
