[Towncar]

You'll NEED two programmed keys to do it yourself, this is standard with all PATS systems.
Be aware that this user programmable mode can be disabled by the owner in cases where it makes sense, like rentals etc.

I'll post links to ALL the pertinent documents from your 2004 FSM.

PATS Key Programming Procedures:
http://www.keepandshare.com/doc/view.php?id=699953&da=y
http://www.keepandshare.com/doc/view.php?id=699954&da=y
http://www.keepandshare.com/doc/view.php?id=699955&da=y
http://www.keepandshare.com/doc/view.php?id=699956&da=y

Anti Theft Alarm Triggering Devices:
http://www.keepandshare.com/doc/view.php?id=699949&da=y
http://www.keepandshare.com/doc/view.php?id=699950&da=y
http://www.keepandshare.com/doc/view.php?id=699951&da=y
http://www.keepandshare.com/doc/view.php?id=699952&da=y