I'd try a plastic clothespeg with the end of some insulated wire wrapped around one side, so it will be pressed agains the plug lead insulation for capacitive coupling when the peg is clipped on.
Add a fairly high value series resistor (10K?) from the wire and a couple of inverse parallel schottky diodes between that point and vehicle chassis ground.
Connect the scope across the diodes, ground to ground.
That should hopefully give a signal you can trigger the scope off safely?
If you want to view the waveform, try chains of several diodes rather than single ones each way so it clips at a somewhat higher voltage, and add a pot across the diodes to adjust the attenuation. You may need a rather higher value series resistor and a fairly high pot value, to minimise waveform distortion through the very low value capacitance of the clip on coupler.