Morrib,

Here's another theory:

If one light bulb burned out in you house, would you replace all the light bulbs?

Same goes for glow plugs (GP). Just because one goes bad is no indication that the others will. I would suggest buying 2 and save $45. One to replace the bad one and a spare to put in glove compartment. Remember, GP are not show stoppers. When one goes bad, the only result is miss firing on one cylinder for a short time after starting. Once the engine is runnng, the GP are essentially inert.

If it ain't broke, don't fix it. If it still works, don't replace it.

P E H