It varies by bike. Some have continuous tubes, or (like mine) sort of funnels to catch and guide the wire to the exit hole. Problem is that you won't find out how hard it can be to thread a wire until it's too late.

Unless I'm sure that it'll be easy, I cut the existing cable to get a clean end at the front, and use a piece of heat shrink tubing to splice the new wire to the old, and gently push/pull through. Use a decently long piece of shrink tube so you have some margin in case you pull a bit too hard.