There are various ways to rout the cable at the derailleur attachment, but only one right way. If the cable is on the wrong side of the bolt the shifting will be off and can drive you crazy. Any tight turns or kinks in the housing can cause excess friction and poor shifting. If the old housing have the correct length, then cut the new housings the same length. The short housing found at the rear derailleur is subject to dirt and water that will cause excess friction and shifting problems. It needs to be replaced. The length and curvature are critical due to its short radius curve.