I'm pretty sure it was a camera and microphone, not a light. This was how the Chief was supposed to keep an eye on their missions while he was back at headquarters. That idea may have fallen by the wayside when the stories had him taking a more active role and not just staying behind. Anyway, it's not built into Cliff, he's simply got it strapped on his chest; I remember at least one story where the Chief wore it while a damaged and immobile Cliff watched the action on the screen.