OK.. Lets look at how the ground circuit operates (in most cases, cant tell for sure without a wiring diagram for your machine) - When operator is in seat, opening the seat switch, PTO disengaged, clutch pedal depressed, and key to run position, all circuits to ground for the grounding circuit are open. If operator leaves seat with the PTO on, or with the parking brake NOT set (which typically holds the brake pedal down) the ground circuit will complete through either or both switches when the seat switch closes .
So.. Basically you need to first test making sure operator is in seat, brake depressed (for starting) and PTO switch off. If there is continuity to ground, one of those switches is not working properly. If you sit in the seat and let off brake pedal, there should be NO continuity to ground (likewise with PTO switch on) because the seat switch interrupts the ground circuit. If that is not the case, seat switch is bad.
If continuity to ground is gone with operator in seat, then everything *should* be working normally. - if not, then seat switch is still completing a circuit somewhere, or it is shorted, or one of the wires is shorted.
Its difficult to be accurate without any wiring diagram information, though.. if it comes down to it, I can opt out and maybe a Canadian expert can take it over, if they have access to wiring diagrams for that model.