Glow plug seem to be like light bulbs, I don't know what the mean time to failure is but they just randomly fail at the least opportune times. I'm not surprised that you have a dead one after 20k+ miles. Most folks here would not recommend Autolites, I have had good luck with Bosch.

I have the Bosch GP relay with the afterglow feature in my 300D. It seems to help the engine run smoother, with less smoke for the first couple of minutes.