i did it this way myself, i will take a pic later and tell you which fuse i used, it does really matter though as long as you use 1 of the big fuses, a camera will draw very little ampage
oh if i recall, i used a fuse with a permanent feed
have you looked into just wiring it into the mirror housing and using the ignition live from the rain sensor and just a screw into a screwport already there for the earth?
Oh right that's a shame, is there defo not another wire you could tap into, interior light or auto dimming mirror or the little blinking LED linked to the alarm maybe?
Auto dimming mirror is probs the only ignition live out of them 3 tho I would have thought?
If you're struggling to find a good earth, it's not too big a job to take the glovebox out entirely. That'll allow you a much better view of what's behind there. There's also an earthing point in the passenger footwell hidden away bends the kick panel / speaker cover.
I earthed mine using a screw that's hidden behind the passenger air vent. I think it holds the rear of the dash to the car.......ill see if I have a pic.