OK,
I've been perplexed with my Cinq overheat warning light so did some experimenting. The manual says it only lights when overheating. Tested that it worked OK earthing the sensor wire so it must be a switch/wire/bulb (only has one wire)
The Seicento owners manual say it lights up with the ignition and then goes out (like the code light) I believe that is managed by the ECU some way and has a resistance at all times?