How are you getting power to the glow plugs without full relay function?

I'm not familiar with that glow relay part number but I'll assume the connectors are compatible with MB passenger car harnesses and it's an afterglow relay. Forget the wire colors and check for pin numbers either on the relay itself or on the connector. The numbers should correspond between relay and connector pins. Here's the pin list for a 90-91 300D 2.5:

pin 1 = +12V when key switch in run to initiate preglow cycle (W124 = hot side of fuse 7)
pin 2 = +12V when key switch in start to terminate preglow cycle (W124 = NSS)
pin 3 = +12V to gauge cluster glow indicator
pin 4 = ground
pin 5 = blank
pin 6 = signal from glow temp sensor by water pump to determine preglow and afterglow cycle times

Assuming each glow plugs test between 0.7- and 1 ohm at the glow relay connector, leave pin 2 disconnected then see if you get the max preglow cycle of ~30 seconds. I don't know how the glow relay behaves when there is no temp sensor.

If this gets to be too much work, a 617 pencil type glow relay should work for you since afterglow isn't much of a benefit in the tropics if you have healthy glow plugs.

Sixto
87 300D