It might be this one. It's one of my faves. You should check it out even it's not the right fic.
Wicked Ways by noaluvjames
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=13282&warning=NC-17Summary: It's punk princess Buffy with a snarky attitude and uptight Spike, a wall street broker who used to be a rebel (also answer the name of William). Buffy's stepbrother, Xander a pub owner is too lazy to drive her to college, a day's drive away. Choosing the easy way out he asked his old pal Spike for a favor...only things are not as easy as they seem. Will snappy Buffy drive reformed Spike completely insane or bring out the best in him? ... A Spuffy road trip with a twist.