you could try dumping the content of the payment request to the screen instead of actually submitting it
/includes/modules/payment/authorizenet_aim.php
around line 376 you have:
change that to:
Code:
echo '<pre>' . print_r($submit_data, true) . '</pre>';
die('halted - payment request NOT submitted');
$ch = curl_init();
and then run some test transactions to see what you're sending to Authorizenet.
Beware -- the CC data you entered will be displayed on the screen along with all the other data prepared for sending for approval.