How does one access old private messages? I have a number of "stored" ones but cannot view them or delete them.

There is a link to your Private Messages near the upper right corner of each page of the Shop Forum. It's below where it says Welcome, username.
Here's a direct link (http://www.mercedesshop.com/shopforum/private.php).

Once you go there be sure to check your Inbox and your Sent Items.

The problem is that there are old messages that are not viewable. For example, at the present moment, the bar graph on my private mailbox states there are 13 messages total, but it only shows 3 in the Inbox and 4 in sent mail. So where are the other ones?

-AC

It is possible that your mailbox is corrupt. I can fix it but it will delete all messages in your inbox and sent box.
Send me a PM if you want me to do it.