I have an Australian delivered 99E430 Avantgarde. In the owners manual, there are instructions for a garage remote tuning option for the interior rear vision mirror on some models. I know mine doesn't have it, but can I get it by just buying a mirror from an overseas model? Would the wiring be there behind the original or do I have to replace a whole heap of other stuff just to get this function on mine?

 

The Homelink system is all contained in the mirror. I don't know what wire MB uses to power it, but it will only need one and a ground. It was stock on my C320, and only operates with the ignition on. There is no draw when the buttons aren't pushed, so it would be fine to wire it as a constant-hot.
I've after-installed a Homelink sun visor on an Eagle; on that car it merely replaced the visor with an illuminated vanity mirror, using the same Molex plug. It is something of a drawback to arrive in the evening with my hair inadvertantly mussed, but I'm doing my best to cope with it. At least I can get indoors more quickly while evading the papparazi.
I use mine with an X-10 unit on the house to turn on the porch lights.