From what I gather, it seems the official megane wiring diagram is a program that needs to run on a PC or whatever and covers a ton of different Renault's and you need the entire program to make it work rather than it being a dozen or so pdfs to view, hence it's lack of appearance on the wider net.
Or a Haynes manual but unfortunately no one seems to have scanned it and made it available, not for free anyway, I don't reckon you'll have a choice but getting a Haynes jobby if you really need it. But then I never resented paying for a Haynes manual myself in this time we live in. They don't seem to do one for my current car which is a shame.