Dash Cam Power

[WiNgNuTz]

So i'm looking at investing in a dash cam, but I dont want a wire running all round the cabin, down to the cigarette lighter, so i was thinking of tapping into the switched live in the rain sensor (with the idea of having the camera behind the rear view mirror). If I was to cut the end off the supplied power lead, I would assume I'd have a simple live and neutral wire within, which could be spliced into (from cullys auto dimming mirror guide) pin 7 (ground), and pin 3 (ign sw +12v) on the rain sensor.

Anyone else had any experience doing similar, or can see something drastically wrong with my thinking? Just not keen on the idea of buying a camera, lopping the end off the power lead, then finding that I'm wrong.