I think you have solved your problem. Buy decent plugs. Ream the holes. Meticulously clean all the electrical contacts during installation with fine sandpaper or steel wool. I use electrical grease on the contacts to ensure a good connection over a long time. Don't over torque the new plugs or you will break them. As an aside, there isn't a fuse for the GP system.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).