On my 84 300D relay always has Battery Voltage at the Strip Fuse and the Flat wire that supplies the Battery Voltage.
However, the Glow Plugs only get power when the Key is in the Glow Position and when your are Cranking the Enigine.
Ounce the Engine is running the Glow Plugs get no power.

Some of the Older Glow Plug Setu-ups have a Coolant Temp Sensor that is connected to the Glow Plugs. I am not sure if this keeps the Glow Plugs on until the Coolant Temp is up or not.

The several years newer models have something called Afterglow that indeed keeps the Glow Plugs on for several more minutes; but eventually the all turn off.

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84 300D, 82 Volvo 244Gl Diesel