Automotive electronics form a system that has been iteratively expanded into a complex architecture. Without having access to manufacturer data, insights into the functional end electrical behavior of the subsystems and their interdependencies inside a vehicle remain unclear. This makes it difficult for research to develop sophisticated optimisation solutions for energy consumption and computing power to accelerate the environmental impact reduction of the automotive sector and to fulfill governmental regulations. Existing measurement systems are either expensive or lack measurement granularity, what restricts accessibility to data and results in assumption-based contributions. This work presents a novel low-cost, modular in-vehicle sensor system designed to monitor the energy consumption of automotive auxiliary systems. The novelty of this approach lies in breaking down the power consumption to the level of components (Electronic Control Units (ECUs), sensors, and actuators), while supporting correlation with vehicle driving parameters. This approach allows users a wide range of applications for analysing the computing power of individual ECUs, but also the traceability of control logics by recording the activation/deactivation of actuators and sensors. The recording allows a digital image of the functions to be created and optimised in a simulation and compared with the real E/E architecture. This easy-to-integrate system, powered via USB and using adapter connectors for non-invasive setup, offers six channels to measure up to 18 V and 10 A at 200 Hz (12-bit resolution). Validated in a test vehicle on Park Distance Control and Surround View Systems, it identified how vehicle speed affects sensor activation and ECU power, pinpointing opportunities for idle modes and energy savings.