Here's the wiring diagram for the relay socket... it will need to be connected to a dedicated coolant temperature sensor and the starter feed:The coolant sensor tells it how long to stay on for, and the starter feed tells it to shut off as you attempt to start the car.Note that it's intended to drive the fuel shutoff solenoid via pin 86... you won't need that line since you have obviously figured out a different way to power the solenoid as part of your swap.
Thanks for the suggestion, but there's no water trap idiot light on this vehicle.The issue is absolutely tied to engine rpm, which is why I suspected vibration as the cause. Revs of 2500+ rpm always turn the glow plug light on when the engine is cold. It's just like clockwork. But after the engine reaches normal operating temps, the light (and presumably the plugs) stay off, even beyond 2500 rpms.