Definitely confirm the wiring harness between the relay and plugs is secure.

Also the relay control is not like the older models, there is a 2 wire control connection. One wire is terminal 50 (the starter signal) and the other is a single wire PWM data connection to the ECU (this is is how it talks to the ECU to determine glow time, and communicate dead glow plugs). This terminal is rather flimsy and can be intermittent. Tjts1 had this problem with his glow relay, search for a thread from him on this.

The glow light does not get controlled by the glow relay, the ECU controls the glow light via CAN bus to the cluster. Far cry from the early 115 where the glow light was literally in series with the glow plugs.

Try pulling codes with an OBDII reader, there are 3 codes for glow plug failure, one is cyl. 1 or 2, another for cyl. 3 or 4, etc.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech