How enable outside temp?

[MarkN]

I have an '97 JE0A Espace (standard) without outside temp indication in the display (below the time, as on models with this option).
I wish to add it, by mounting an outside temp sensor in the left mirror: hole for mounting and wiring (underneath left dash speaker) already present.
Fysically connecting therefore is easy, but how do I make the display/computer know there's an outside temp sensor connected?

Mark N