hi gemma, at the end of your ghds are 2 small screws and a rubber ring, remove the rubber ring then the 2 screws, and you should be able to lift the cap up, then you slide end of new cable in to the end, locate into the socket, and refit the cap, 2 screws and rubber ring,
I hope this helps,
I hope the ghds you got on ebay have internals lot of these are sold on ebay, but nothing inside them, eg. empty shell.
kevin
May 2008