I've never had much luck relying on ohmmeter tests for glow plug analysis. With the pin type, even one that shows good on the Ohmmeter, can have a burned shell or other malady.

Ten years or more ago, they were so cheap I just replaced them when there was any doubt. With the later engines they are more difficult to get at and they are expen$ive in todays parts world.

I'm not sure about the 124 cars, but on the 123's with pin type plugs, number one plug served as the sample for the glow plug relay mechanism. The current measured throught hat one indicated if the system needed to continue to glow or not.

Hope this helps.