Other than an offhand reference in Johnny Spinrad’s biography about Spinrad Industries winning the bid for the French government's “Monaco Reconstruction project” and somehow parleying that into a fifty-year lease on the principality… I don’t think Monaco has ever been mentioned (let alone detailed) in any Shadowrun sourcebooks. Seems like it would be a great locale for some black trenchcoat style games though.
But, I dunno, the weather’s always warm and sunny. Can you have cyberpunk without perpetual rain?