You have the best idea if you want to keep the system as stock as possible.

Your idea is a better option than buying the Glow Plug Upgrade Kit that comes with the Special Glow Plug Relay because if the Kit Relay went bad; where would you get a new one and a what cost?


Some what in between what you mentioned the Ford Starter Solenoid could be used in conjunction with the same circuit and Wire that would turn your Old Stock Glow Plug Relay on.
That would allow you to use your Key in the normal Glow Position but the Glow plug Light would not work. (The light could be wired in but would not turn itself off.)

If you wanted the Solenoid to be on while the Engine is cranking as it does with the stock Glow Plug Relay it could also be hooked up to the Violet (Purple?) wire that connects in your case to the Starter Motor Solenoid.

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84 300D, 82 Volvo 244Gl Diesel