How to return JSON data using Drupal 7
To return $data in json format using Drupal-7's page callback output, you should return through :

drupal_json_output($data);


in your page callback function.
For more ref : http://api.drupal.org/api/drupal/includes!common.inc/function/drupal_json_output/7

Note : in your javascript in car.module, var pageUrl seems undefined. Make sure it is defined.