[CadillacSTS2003]

hyber im gonna say likely not
your car needed cell phone equipment for that years onstar
even if it did have onstar the vcim used had 4 plugs and was controlled via the hand held unit (ie the phone) not an overhead mirror control

im sure using all the wires used in all the systems you COULD get the module correctly wired in but then you have the issue of interfacing it with the controller in your case the impossible to find onstar cell kit