Not that simple.

To make TMC work you need:

1. A map disc with the TMC database on it. The database references the broadcasted TMC location IDs to real lat/long positions on the map. Who holds the master TMC location database for Australia? Probably the TMC-provider. And they won't give it to you for free...
Then you have the problem of compiling a new map CD with the new TMC database... I don't know how easy that is. Maybe just add the new TMC database file? In what format? Maybe integrate with the map data somewhere...


2. You also need to change the nav software to decrypt the TMC message. To do this you need to get access to the nav source code, and know the encryption/decryption used on the TMC. The nav is written in OS9000, which is now very very old. You would need to reverse compile the OS9000 source code, understand it, rewrite it, and recompile. You thus need to be an expert in OS9000. And you also need a good source of Mk4 navs as the chances of corrupting the flash and killing the nav are extremely high. Or else you need the ability to reflash the nav manually.

So whilst nothing in this world is impossible, some things are simply very, very hard...