Montague

I often use the inner part of a nylong tube-in-tube pushrod, such as the Dubro Laser rod or Sullivan Nyrod. I shove it in from the tail end, push/pull to where the servos are, then thread the end of the pushrod into the nylon, and pull back through.

It does depend on the exact setup though.

I've used string to pull wires though wings and fuses before. To get the string in there, I find it helps to tape or tie a small nut to the end of the string, then hold the wing/fuse vertically and shake, letting the nut find it's way through the structure to where I need it.