How do I clear the message waiting light on a Panasonic KX-T7668 telephone when there is no message and the light is staying on

Not true, there has to be a message or it wouldn't be lit. Try *700 and your extension on the intercom. It clears *all* messages from everybody to that phone.

In the family of 76xx phones, you can also press the MESSAGE button and look at the display. It will tell you who the message is from. On the phones with soft keys, one of the buttons will be the CLEAR feature.

Carl