Should replacement of the relay always accompany replacement of the glow plugs? Under what circumstances would you replace the relay?
Also, is a "reamer" essential to the process?- looks a little pricey!

You should not have to replace the relay unless it goes bad. Search around - you can usually find a good, working used one.
Reaming out the holes is something that should be done if you have the tool. I would not lose any sleep over it if you are not able to ream them out.

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Jim

I remember reading somewhere, probably here, that you can use a 6mm allen wrench to clean out the glowplug hole.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member