Don't Let it Happen Twice

"Don't Let It Happen Twice" or something like that, or something about once is an accident, twice is a habit, was the title of the fic, I remember reading it YEARS ago, maybe six or seven years in the past. I have no idea who the author was or even which site I read it on. It's a PWP type, that starts out with Spike and Buffy having sex in the DeSoto (or on it, maybe), with Buffy telling herself it can only happen once. Which means it happens again. Sound familiar to anyone?

It sounds like Don't Happen Twice by Cobweb and then Paradise By the Dashboard Light by Cobweb
you can find them here
http://www.fanfiction.net/u/70488/Cobweb