We don't have sites specifically based on car sex, but
www.terryvision.net has some car sex scenes. It's primarily a public nudity site with some scenes that include driving from place to place and action in the car but there are many more.
Here are a couple examples:
http://terryvision.net/index.php?id=42#update
http://terryvision.net/index.php?id=55#update
If you're interested in promoting it, I can compile a whole list for you with every car sex scene - just let me know.