I read this fic for very long ago like years ago. All i remember is that Buffy and Spike start working together and feelings start to develop but Dru comes to town and Buffy and Spike are forced into a show down with her and Spike ends up staking her to save Buffy. After that he takes off and packs his stuff ready to leave Sunnydale for good but Buffy jumps on the hood ofhis car i think or maybe just in front of it to stop him and tells him she loves him.

Ok, this is probably so not the story you are looking for, but it reminded me of Divinity by Sass Angel (?). It's Spike/Tasha in this story and Buffy jumps in the car and takes off. You might want to check it out. I'm not good at posting links.