Possibly related but not guaranteed.
Might be grounds if related, clean all ground contact points.
A voltmeter is your friend here.
Each glow plug should measure very low ohms (like 3, 4 ,5) when measured individually. Naturally 12 volts needs to get to them to work.
To measure them correctly U need to disconnect them from their 'buss bar'.
Ignition key switch also might be the cause and often due to bad internal contacts, A quirt of WD40 will help soften up the dried lubricants inside that are often the cause. (hard dry lube prevents contacts to mate)
Also on some systems the plugs do heat up but the dash indicator does not glow. If after a heat cycle plugs are warm to touch they plugs are OK.
Another 'test' is a 12 volt battery will read just that but while plugs are powered up voltage will drop to about 9 volts or so during activation.
A good battery will then shortly return a reading of 12 volts shortly.
