Auto Fill di Php Ajax

Selamat siang, ane mau nanya, ane buat autofill di php tapi gak berfungsi. ane udah cari2 masalah nya cuman gak nemu2 
ajax.php

<?php
include '../../koneksi.php';
if(!isset($_SESSION)) {session_start();}
$id_plat = $_GET['id_plat'];
$data = mssql_query("SELECT driver,penumpang,plat FROM othe_disp where plat='".$id_plat."' ");
$result= mssql_fetch_array($data);
$array = array(
            'driver' =>  $result['driver'],
            'penumpang'   =>  $result['penumpang'],);
echo json_encode($array);

?>

index.php

<form  action="">
	<div class="form-group row">
	<label for="input text" class="col-sm-2 col-form-label">No. Plate</label>
	<div class="col-sm-6">
	<input class="form-control" type="text" name="id_plat" id="id_plat" >
	</div>
	</div>
	<div class="form-group row">
	<label for="input text" class="col-sm-2 col-form-label">Driver</label>
	<div class="col-sm-6">
	<input class="form-control" type="text" name="driver" id="driver" disabled>
	</div>
	</div>
	<div class="form-group row">
	<label for="input text" class="col-sm-2 col-form-label">Passenger</label>
	<div class="col-sm-6">
	<input class="form-control" type="text" name="penumpang" id="penumpang" disabled>
	</div>
	</div>
</form>
<script type="text/javascript">
	function isi_otomatis(){
                var id_plat = $("#id_plat").val();
                $.ajax({
                    url: 'ajax.php',
                    data:"id_plat="+id_plat ,
                }).success(function (data) {
                    var json = data,
                    obj = JSON.parse(json);
                    $('#driver').val(obj.driver);
                    $('#penumpang').val(obj.penumpang);
                },error:function(jqXHR, textStatus, errorThrown) {
		          alert('Error : '+jqXHR.status);
		        }

                );
            }

</script>

Kalo datanya sudah di lempar dengan json_encode, mestinya di ajax nya nggak usah pake JSON.Parse jadi langsung aja.. Misalnya data.driver