Slechte lasverbinding in een naderhand aangebrachte installatie

Something was added to the vehicle and connected by cutting into its wiring rather than through a designed interface — a tracker, a dashcam, an insurance box, a towing loom, a stereo, a light bar, a fleet terminal, a converter's body equipment.

The join itself is the fault, and there are only a few ways it goes wrong. A twisted-and-taped connection that corrodes and turns intermittent. A vampire tap that pierces the insulation and severs strands. A signal taken from a data bus pair, which is a matched, terminated transmission line rather than a wire — anything hung on it changes its electrical characteristics and can corrupt messages for every module on it, not just the one nearby. An earth made to painted metal or to a different point from the module it serves, creating a voltage difference where the design assumed none.

Two consequences recur, and neither is obvious from the installation. A device that keeps reading the network when the vehicle is parked stops the vehicle sleeping, and flattens the battery over a few days — normally answered with a new battery and then a new alternator. And a splice near safety-system wiring is a serious matter, because tow bar looms and audio installations are routinely run through the areas restraints sensors and their harnesses occupy.

So the question that finds this is not "what is broken" but "what has been added". It is worth asking about work a previous owner had done, and about anything that seemed too cheap for the job. The proper repair is to remove the splice and reconnect through the vehicle's own interface, which usually exists and was skipped because it costs more.