This is very true, the bottom line is that your OBC expects to see a certain resistance across the tail light circuit. If the resistance is substantially higher or lower than the preset, it triggers the warning. With this in mind, all you have to do is consider what factors could introduce changes that cause the resistance to change - burned out bulbs, bulbs of incorrect wattage, dirty trailer connector plugs, corroded bulb sockets or wiring loom connections, faulty brake light switch (which may actually function perfectly) and wet or damaged wiring.