you'll see almost the same thread title here
here's a shorter answer to your question
if you want to do a top job, you'll have to take the ceiling skin down. It's actually not too bad a job, you can do it alone--the trick is to get the skin from concave to convex along it's entire length, then you can pull one side from the rail. You'll find all the wires up there, then they branch off and go straight down the wall.
Taking down the larger side skin is more difficult--lots more rivets and they are difficult to open partially because they have such a concave shape. You can get a little opening at the ends, but you'll find that it's easier to take out all the rivets along one edge (top or bottom) and curl it a little so you can get your hand inside. I recommend the bottom edge, since you'll be able to thread wire behind it from the top.