Mesh collide, terrain & object

Hey, I have complex terrain mesh and complex car object, I cant use any other meshes on the car/terrain as they are offset and strange. Ive used mesh collider on both (on every part that made up the whole object), they both (the car and terrain) interact perfectly fine with the player which at the moment is capsule collider, though - when interacting with each other, the car simply pass through the terrain… what am I doing wrong?

Is it once you play it? And does it fall through? If so, Make sure that when you start the game the colliders are above the terrain… if they are crossing over the car will fall through.