Find in automatic_addressbook.php:
if ($rcmail->config->get('use_auto_abook'))
and replace with:
if ($rcmail->config->get('use_auto_abook', true))
Cool pointed me in the right direction. I found that this didnt quite work, as the plugin checks the setting in lots of places, eg in register_recipients function when sending mail, etc .
So for anyone else trying to do this, what I did was I had to replace every instance of
$rcmail->config->get('use_auto_abook')
with
$rcmail->config->get('use_auto_abook', true)
apart from in the function save_prefs.
Cheers,
Menno