That is only the controller. The relay that is triggered by it is elsewhere. You will need to manually trigger your contactor if you don't trust the controller. The relays aren't known for sticking on.I am a bit puzzled. I have a 94 E350 van, 7.3 L N/A, glow plug controller at the rear of the engine, easily accessible via removal of the dog house in the cab.
I flat do not trust that relay.
Leave the controller. Replace the relay if you want. Run a manual push button to the relay.