Modify Ad list in admin

I'm trying to add a "group name" column to page "/admin/content/ad".
I almost have it but it's not displaying in the column, but under the table.

In ad.admin.inc in function function ad_admin_ads(), I added this:
$form['groupname'][$ad->aid] = array('#value' => _ad_get_group_name($ad->aid, $ad->vid));
under the line:
$form['group'][$ad->aid] = array('#value' => _ad_get_group($ad->aid));

I gets the Ad group names correctly but it's not displaying in the table correctly.
Is there another place where this table gets rendered where I need to add another column header?

thanks