You have a global var car and a local var car
var car;
function init() {
// THE USUAL STUFF, scene, camera, renderer
}
function addSceneElements() {
// Sphere
sphere = new THREE.Mesh(new THREE.SphereGeometry(8, 70, 20), blueMat);
sphere.position.set(-260, 9, 125);
scene.add(sphere);
var loader = new THREE.JSONLoader();
loader.load( "models/hotride.js", function(geometry){
var material = new THREE.MeshLambertMaterial({color: 0x66CCFF});
var car = new THREE.Mesh(geometry, material); //// <<<< local var !!!
That means that the global car won't have the mesh assigned to the local car. Just remove var in the line !
car = new THREE.Mesh(geometry, material); //// <<<< global var !!!