307 starting fault

Hi all. My 02 307 has difficulty starting. I thought it was the glow plugs as it seemed to b when the colder weather came. So replaced them and still the same. So replaced the relay. But it won't energise? Have a live and earth but don't seem to have a signal from the ecu? I think this is what that wire is for? I still think it is a pre heat problem. So would be grateful if anybody could help. Or tell me what each wire should do?
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