Very strange indeed. I can only wonder about what that mechanic is confused about.

What it almost sounds like is a marginal connection. That when trying to make the circuit. A small amount of current may be flowing. Slowly heating the component until it expands enough to eventually make better contact.

Since it also is a simple series circuit. When it will not work it should be very easy to locate the issue. Maybe the mechanic and you could have a talk or print off this post and take it to him.

He for example could be confused by never running across a glow plug system in series. Or has not realized this one is. Plus as Tom mentioned there is a basic switch that apparently has a known failure rate in this older system.

Why I say strange as well. Is if he is coping with modern electrical systems in cars. This should have been a walk in the park for him.

Even with my extensive electronic and electrical background. Very modern cars electrical systems in cars do not make me happy. I might feel better about them if I worked on them a daily for a living.

In fact I almost see a time coming where some forms of electrical failures in cars will mean they are not worth repairing. Intermittent conditions especially.