You can easily tap into the 10a horn fuse. That one has enough space for a generic mini fuse tap. For getting into the cabin, I’ve used the grommet you were talking about on the passenger side. Remove the glove box for access. Feed a wire through (it will be tricky) and you are set. Use the mounting bolt that’s on the shock tower for ground using an eyelet connector. Not sure about a full 100w, but it easily handles a few LEDs in my RAV. Hope this helps.