Vehicular electrical wires and communication systems can be affected by faults. Depending on their severity, faults can be divided into two families: hard and soft faults. Hard faults indicate open/short circuits that can lead to serious breakdowns because they prevent the flow of energy or information. Even though soft faults do not have such severe impacts on the system, they may develop into hard faults in the long term, hence the need to detect them. In this paper, an unshielded twisted pair cable that undergoes a water-tree degradation is considered. A soft fault, which may correspond to the effect of a mechanical constraint on the cable, is introduced as a series resistor. The studied network is a tree-shape network composed of one source and multiple receivers, one at each end of a branch. Assuming that these receivers operate in a healthy state, to detect the fault, the transfer function from each receiver are monitored and a detection index is used. Another index, based on the comparison of the effect of the fault at each endpoint, is proposed to locate the affected branch. To summarize the detection and localization algorithm, a signature matrix is generated. Simulation results are presented to illustrate our approach.