How do you obtain the Slamvan in the TBoGT mission Frosting On The Cake

Do the mission regularly until you reach the cutscene at the docks. Whether you skip the scene or not, start pressing buttons frantically until you ''exit'' the vehicle. You will still be in the lock-on mode so switch to a melee weapon to return to third-person view. Hide behind cover until it fails the mission for abandoning the limo. Steal the Slamvan and bring it to the parking space of your choice :)