On the 648 (and 642) the Glow Plug Relay talks to the ECU on LIN bus (single wire slow speed serial) and shares that line with the voltage regulator.

This is the routing of the data wire from the ECU to the glow plug relay and alternator.
All of these are 0.75 mm (or about 18 gauge American) solid blue according to my Star Finder

Pin 66 on the ECU, goes out to a Splice Z26/1, in the harness near the EGR valve
Branches to Pin 1 on the alternator regulator connector and Pin 12 on the N14/2 glow plug relay

On the GP relay multi wire connector
pin 1 cylinder 1 2.5 mm (similar to American #14) black/blue
pin 2 cylinder 2 2.5 mm black/violet
pin 3 cylinder 3 2.5 mm black/red
pin 4 cylinder 4 2.5 mm black/yellow
pin 5 circuit 87 (protected engine management logic power) 0.5 mm (#20) red
pin 6 circuit 31 0.5 mm brown
pin 7 cylinder 5 2.5 mm black/green
pin 8 cylinder 6 2..5 mm black/white
pin 9 , 10, 11, no connection
pin 12 LIN bus 0.75 mm blue

The big connector with single wire on the GP relay is circuit 30, straight battery feed, via a 10 mm (or about 6 gauge American) via fuse 69, 125 Amps, in F32 (the 'front prefuse' box in the passenger foot well behind the shielding plate).

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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