In addition to the missing single quote, you failed to put your closing parenthesis )
Also, you can do it how you like, but I took the liberty of rewriting the SQL to concatenate a bit cleaner, and to use concatenation on the $_POST variables.
$query = "INSERT INTO ads VALUES('".$_POST['ad_id']."', '".$_POST['contact_name']."', '".$_POST['contact_phone']."', '".$_POST['contact_email']."',";
$query .= "'".$_POST['year']."', '".$_POST['make']."', '".$_POST['model']."', '".$_POST['vin']."',";
$query .= "'".$_POST['price']."', '".$_POST['exterior_color']."', '".$_POST['interior_color']."', '".$_POST['transmission']."',";
$query .= "'".$_POST['doors']."', '".$_POST['cylinders']."', '".$_POST['body_style']."', '".$_POST['ac_front']."',";
$query .= "'".$_POST['ac_rear']."', '".$_POST['airbags_front']."', '".$_POST['airbags_passenger']."', '".$_POST['alarm']."',";
$query .= "'".$_POST['alloy_wheels']."', '".$_POST['abs']."', '".$_POST['bucket_seats']."', '".$_POST['cassette_radio']."',";
$query .= "'".$_POST['cd_changer']."', '".$_POST['cd_player']."', '".$_POST['cruise_control']."', '".$_POST['fog_lights']."',";
$query .= "'".$_POST['leather']."', '".$_POST['memory_seats']."', '".$_POST['navigation_system']."', '".$_POST['power_locks']."',";
$query .= "'".$_POST['power_seats']."', '".$_POST['power_steering']."', '".$_POST['power_windows']."', '".$_POST['premium_sound']."',";
$query .= "'".$_POST['rear_defroster']."', '".$_POST['rear_wiper']."', '".$_POST['keyless_entry']."', '".$_POST['sunroof']."',";
$query .= "'".$_POST['third_row_seats']."', '".$_POST['tinted_glass']."', '".$_POST['tow_package']."', '".$_POST['tv_vcr']."',";
$query .= "'".$_POST['additional_selling_points']."')";