On the 617s with Pencil Glow Plugs you always have Battery Voltage at the Strip Fuse and the Flat Copper Strip/Wire.

Some say that the only 100% check of the Glow Plugs is to pull them out and use a Jumper Cable to actually Glow them using the Battery. Ounce in a while the Ohm out OK but are still not working correctly.

While the Glolw Plugs are out you could Ream or otherwise clean the Carbon out of the Glow Plug Holes.

You did not list the milage on your Engine.
If the Glow Plugs check out OK and the rough Idle is not being caused by an Air Leak (or another Fuel delivery issue) what is left is Injectors or a issue with the Compression.

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84 300D, 82 Volvo 244Gl Diesel