Ok, gave this a go, admittedly on a Clio, not a Megane. 1. you can't reach 2 of the release clips for the housing, so scratch the clip nearest to the body of the car, at the mirror hinge as it were. Third one is behind the mirror mechanism, and so inaccessible without removing the whole mechanism, but luckily didn't break. 2. The sensor is on the driver's side, at least on a UK car, so scratch the second inner clip taking that housing off. 3. The sensor is not prise outable, it's held in with a star shaped nut. 4. the wiring is grouped with the wiring for the mirror moving mechanism with a covering plastic sheath and I'm not getting into cutting tight wires and trying to solder/twist/heat shrink anything like that. Took photos but can't see a way to add them to this post.
Caroline, January 2016