I think you have it wrong. I get:
Top (labelled positive in) - GND
GND
+12v
Bottom (labelled Ground in) Signal
Reasons:
It would make sense to cut power rather than signal with the trigger switch. Also I'm guessing that the component it runs into (hiding under big coil) is a power diode for reverse polarity protection. (It would be pretty inconsiderate to not have reverse polarity protection on a device that uses clamps for power hookup) The idea would be no current would flow through the diode if hooked up backwards by mistake.
The bottom one runs through some high value / low wattage resistors setup as a divider into what looks an awful lot like a trigger circuit, and nothing else.
If you find an inductive pickup for it, you may have to "tune" those resistors for the right sensitivity.
"Grounding" the case through that resistor makes some kind of sense.