Just a quick correction, you'll want to tap into the rear speaker feed. When I installed mine I took the feed from the fronts and it doesn't pick up the signal from them, you have to use the rear.
In terms of a constant feed, I'm unsure about the fuse you're referring to, but I also took my feed from the red cable feeding into the fuse box as mentioned. In the passenger footwell, remove the upper cover held on with 3 T25 Torx screws, then look to the left. You'll see a thick red cable that leads upto the fuse box. I cut away the plastic sheathing and coiled & taped my constant feed onto it, then re-wrapped and taped up the cable: