Please note: Some information on this page may be in German!
The sense of this circuit is that an LED lights up when any signals are on the CAN bus. This can often diagnose faults quickly because you know, if there is any communication on the bus system. Unfortunately, I don't have the built-up circuit → that's the reason why I can't show you any pictures.
Here you can download the circuit diagram in PDF format: Circuit diagram: CAN activity indicator [German]