On the otherhand an old intermitent glow plug relay is far from unknown. Use a voltmeter as mentioned to establish one way or another. If thats what it turns out to be a used unit from a salvage yard shoiuld get you by.
Very few 240ds are scrapped because of a bad relay in my opinion. Remember that watching the ceiling light as an indicator is a good ideal as well. When the glow circuit is activated the light should dim a little until the relay opens up again.