The Soap Box Car is required for completing the "Endless Runner" feat. Complete all feats to get the "Feat Fetish" achievement. It can be found at the entrance of Cima Leon: Centcom (N 40 41.756, E 5 43.986), which is on the island where the game starts (Insula Fonte). You must play through Act 1 of the story to disable the airstrike FOW or else it will destroy the Soap Box Car. Go to the "Feats" section of the Leaderboards to view the feats you have left to complete. Everything else is fairly easy and should come during natural gameplay. Note: Feats are disabled during story missions and can only be done in free roam.