The outside light fixture was replaced on the brick wall of the house and now there is a hole between the new fixture and the brick? How do I seal it? Here are a couple of pics to show what I'm talking about:
You could also put a mounting block behind the light fixture. If you can only find one for siding, you could probably trim it with a utility knife to make it lay flat. You can paint these to match the fixture since it is a simple gloss black finish.
I would remove the fixture and find something that's a good fit to shove it the hole to use as a template base for the mortar like a red Solo cup! Hold it up and smear the mortar around to create a nice and even ring of mortar. After it cures reinstall the fixture. Done