Check the decoder ring but that light is controlled by the UCH and as its flickering whilst the engine is running i'd be supprised if its the key code not being read as the car would prove difficult to start. I'd say its most likely a UCH fault (no doubt due to water/moisture ingress) and i'd check there aren't any other electrical issues hiding.
Check all windows work, central locking, indicators, auto-lights, etc... all the things you take for granted that the UCH manages.
Mick