Give me a few minutes to go look under my hood. I suspect that wire you got sparks from is the power feed to the relay contacts. When the relay control closes them the high current goes through the 80 amp fuse to the glow plugs. Will add to this post in a couple of minutes. I personally dislike looking at key switch schematics.

Okay then It was normal to get a spark to ground as the relay contact feed power is present with the key off on the 10mm nut line. Here is your common sense test. Put the test relay back in but leave the smallest plug disconnected. Use a test meter or test light to verify the glow plugs are not getting power. There will be none if that relay was properly operational in your other car.

Next while monitoring the glow plugs for voltage plug in the small plug. With key off there should be nothing and thats exactly what I think you will find. Then turn on key to glow circuit activation and while still monitoring the glow plugs wait for them to go through their cycle and shut down.All should be well. Their is little risk for your substitute relay by observing.

A lot better than getting involved with key switch circuit problems as well. Now if that substitute relay will not turn off after an appropiate time that you can probably guess from experience the key switch circuit does become a suspect.

Electrically by following this proceedure there is in my opinion no risk that I can think of to your substitute replacement relay. The relay contacts in our glow plug relay take a beating with time. Since the relay coil will generate heat if constantly on a mess inside of your old relay is to be expected. Or if the primary control transistor had shorted out it too could have caused lots of generated heat and melted solder. This type of senario I would class as typical on failed glow plug relay type of devices. Let us know how you do.