Are you making up your own looms/cables, or expecting a suppliers to fit 'out of the box'?
If making your own, the simplest way of making them is on a separate layout board, making sure that you get your 'ends' the right lengths before poking them all through the trunking (with a few cms for error!!).
Good luck!!
D...
P.S. Unless you are talking about the original wiring - in which case there is either about 25 metres too much, or (as you rightly point out) you can play a tune on it!!