I was on the highway and I saw a mule's back. It had a platform for someone to jump onto. So I jumped onto its back platform, then jumped onto the roof of it. The back door opened. Went inside, got 2 stars, and the driver got me out of it. Tried it again and it worked. Now I cannot do it anymore. If you guys see one, keep in the back of your head to try opening the back please. I would like these kind of methods to counteract the cap of only having 4 people in a car. It worked twice already and it could be fun in GTAO.