The valley where the garage and house meet, runs into the wall of a raised ceiling
Room.
This wall is brick, with siding on it. I had shingles and rotten board replaced,
But the roof still leaks around this brick. From inside my garage I can see the water running down the brick, then down the wall.

What is the best way to stop this leak?

The best way of stopping this leak is from the outside.

You need to have metall flashing installed to keep the water from running down the face of the brick. Copper.org: Architecture Design Handbook: Architectural Details: Flashings and Copings - Stepped and Chimney Flashings