I live in rainy Vancouver, BC and have been having problems with the glow plugs on my 1977 300D

My glow plug light wouldn't come on and car wouldn't start(It is currently cold and wet).

I found the "first" one in the series had burned out(loop style). Replaced them all because I haven't done it in a while. About a month later had the same problem. Have replaced the first plug a couple of times now.

What could be causing the first glow plug to keep burning out?

Also, should I replace the ceramic insulators as some have small cracks?