Hi

How old is the 406 in question? If its a series one 406 then its easy.
A series two 406 is multiplexed & a little harder but not impossible to
upgrade.

Im 99.9 % certain you should find all the wiring already there hidden
behind the trim.

For the rear windows you would also need the controls on the drivers door
as well as the rear doors, This houses the electric mirror controls as well.

The outside temp display sensor is housed in the passenger side mirror, So
add the pair of electric mirrors & interior trims to your shopping list,
These mirrors will also be colour coded so you will most likely end up
needing a spray can to make them match your cars colour.

The display unit also shows the radio information, But you need the higher
spec Philips radio to work that part, Straight swop but get the radio code.

So your looking for a GLX, SRI or Executive to get parts from in your local
scrapyard, If you find one & your working on a saloon, Get the rear shelf
with the built in storage box & sunblind too, Dont forget the roof clips.