This should be relatively easy, depending on how you're modeling your harness; if it's just a simple cylinder or other profile that needs to sweep along a spline, just create a "sweep along a path" feature in your detail part that will represent your harness. Then, in the deform part dialog, add only the sweep feature to the list of features to deform. In the references list, add only the guide curve for the sweep. Now you're finished in the harness component.
Now, in the assembly you'll be adding it to (where you want to define your installed harness length) create a spline based on your two port locations to define the new harness path. Now add your detail part representing your harness to the assembly; right click on the part & select "Deform". The deform dialog will ask you for a guide for your sweep; click on your spline and click finish. The sweep profile should snap to your spline, and a feature for the deform will be created in your part navigator. I made a quick video that may help clarify.
A couple things to note; you can define more attributes to manipulate in the deform dialog; in this case, I only needed a sweep profile so that's all I added. Also, I've had problems in the past with deform parts not sweeping correctly over splines defined with NX routing (which I always found very disappointing, since routing is so handy for stuff like this). It could be fixed now, it's been awhile since I've used deformable parts in routing.