I might be totalyl off the wall here, but this is my understanding of Canbus...
Canbus (=Car Area Network bus) is the VW version of what all cars have these days.Computers! In the same way the engine is controlled by an ECU that responds to sensors in all parts of the engine and fuel system, the Canbus system links all the interior, dashboard, audio and other 'comfort' components and monitors and controls them.
In the old days, each lightbulb was physically wired back to its switch, which was fed via the fusebox from the battery. With Canbus, the lights are wired back to a lighting controller, which connects to Canbus via 2 wires. The lightswitch panel and stalk are wired to an instrument controller that also has a 2-wire connection to Canbus. When you flick a switch, Canbus detects it from the instrument controller and sends the appropriate message to the lighting controller. In theory, a software re-write could completely alter which switches control which parts in the car without any physical rewiring.
(I used to have a Citroen where the system was known as Vanbus - Vehicle Area Network. After fitting a 3rd-party audio headunit, I gradually found the steering wheel stalk controls for other functions began to stop working because the software was so flaky.)
Canbus measures the current draw of the lighting circuits, on the basis that standard bulbs are in use, so it can warn you of bulb failures. LED bulbs have a much lower wattage and therefore draw a lot less current, which would result in an error. So some LED vendors will put extra resistors in parallel with their bulbs to draw more current and keep Canbus happy. Obviously you then lose the benefit of LED's saving power, but there you go!
