It could be that the relay is becoming intermittant. You can pull it and clean the contacts. The metal cover is removable. If you have series loop plugs like mine, then now would be a good time to clean the linking wire connections with a fine emory cloth. I have had a couple of failed glow-sessions this month, and finally discovered that one of my fairly new loop style plugs (Bosch - made in India - high in the Bavarian Alps) had an intermittant crimp on the tip and I had to replace it. The plug tested open on the bench until I torqued the tip a bit with a flat pliers. Then it tested ok.
Anyway - food for thought.

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'78 300D 310K Maple Yellow / Bamboo
'87 300D 81K Barolo Red / Safron (RIP @ 113K)
2003 E320 34K Pewter/Charcoal
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