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You'll need a differential comparator, something like what was suggested in here. But I'd look for a proper Canbus Rx and extract that to convert to single ended one shot to pulse for activity.
#1: Initial revision
You'll need a differential comparator, something like what was suggested in [here.](https://forum.allaboutcircuits.com/threads/2-led-logic-probe-for-can-bus.119597/)  But I'd look for a proper Canbus Rx and extract that to convert to single ended one shot to pulse for activity.