On my 84 the Glow Plugs get Voltage while the Engine is Cranking.

On the Termial Block infront of the Battery in the most forward part of the Terminal Block is a one White Wire (that comes up from the Starter Solenoid) and a Purple Wire (that goes to the Glow Plug Relay) are screwed together on the Terminal Block.

When you Crank the current comes up from the Starter Solenoid by way of the white Wire and into the Purple Wire and on to the Glow Plug Relay.

I am guessing it is done that way so that if a person is lazy to use the Glow Plug Light and just jumps in the Car and starts it if he/she cranks long enough the Voltage to the Glow Plugs will also heat them and aid in starting. The Glow Plugs working during cranking must be way better than just cranking alone during starting.

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84 300D, 82 Volvo 244Gl Diesel