Hi Ross,
I would recommend using an "add-a-fuse" device, available from Halfords, for the positive feed as that'll save chopping into any wires and give you an additional fused spur which you're wanting.
Its a bit harder to advise on the grounding. It's one of those poke your head round and see if there is a helpful bolt or something somewhere.
My personal preference is always to ground a component direct to chassis / bodywork than to tap into the ground of another component / plug if you can find one. Ultimately though they'll all do the job.
Depending on your cabling routes depends exactly what you'll find. Which way did you go? I'm assuming the dash cam is mounted up top near rear view mirror?
If you've run the power cables down the driver's side A pillar, i think it might be worth popping off the drivers side dash end panel in the door job. I think, from memory, under there is just all metal. Plenty of grounding places.
Note that if you're running a video output from the dash cam that you'll want to run that the opposite side of the car to the power feed, otherwise the power feed will create 'noise' in the video signal.
