Drupal Custom Form Autocomplete. (Syntax of DB field location) - php

Question

I have been working on creating a custom form with an autocomplete field that will search the database for a specific content type's field values and bring back the results so the user can select one for that input box.
My main problem is actually knowing how to target the specific field in the database.
The database information resides in a content type called 'Parts' and within that content type there is a field called 'cross_reference'.
We have thousands of parts and each one has many cross reference values as the 'cross_reference' field is set to unlimited. I need the autocomplete of the custom form to look through these 'cross_reference' values.
Menu and Page
function frontcross_menu(){
$items['frontcross'] = array(
'title' => 'Front Cross',
'page callback' => 'frontcross_page_callback',
'access arguments' => array('access content'),
'description' => t('Front cross'),
'type' => MENU_CALLBACK,
);
$items['cross/autocomplete'] = array(
'title' => 'Autocomplete for cross',
'page callback' => '_cross_autocomplete',
'access arguments' => array('use autocomplete'),
'type' => MENU_CALLBACK
);
return $items;
}
function frontcross_page_callback(){
return drupal_get_form('frontcross_form');
}
Form
function frontcross_form($form,&$form_state){
$form = array(
'#method' => 'post','#default_value' => 'Status Message',
'#id' => 'frontcross_form',
'field_across_reference_value' => array(
'#type' => 'textfield',
'#required' => FALSE,
'#autocomplete_path' => 'cross/autocomplete',
),
'submit' => array(
'#type' => 'submit',
'#value' => t('Submit'),
'#name' => t('Submit'),
'#id' => t('Submit'),
),
);
$form["#submit"][] = 'frontcross_submit';
return $form;
}
Submit
This function puts the form information into a URL query string on the catalogue results page
function frontcross_submit($form,&$form_state){
$options = array('' => TRUE);
$options['query']['field_cross_reference_value'] = $frontcrossval;
$frontcrossval = trim($form_state['values']['field_across_reference_value']);
if($frontcrossval){
$options['query']['field_cross_reference_value'] = $frontcrossval;
}
$form_state['redirect'] = url($GLOBALS['base_url'].'/cross-reference',$options);
}
Autocomplete Function
This is the part that I am stuck on. It needs to look though the database for values that belong to content type 'Part' and its field 'cross_reference' which has multiple values.
I have left the below code as the example I found.
function _cross_autocomplete($string) {
$matches = array();
$result = db_select('cities', 'c')
->fields('c', array('city'))
->condition('city', '%' . db_like($string) . '%', 'LIKE')
->execute();
foreach ($result as $row) {
$matches[$row->city] = check_plain($row->city);
}
drupal_json_output($matches);
}
I hope my explanation is concise enough. Help will be greatly appreciated.
Thanks
Jo