Originally Posted by 300D-91

funola I understand what your saying and I understand the difference between a open circuit (no continuity), a short (resistance value of 0) and expected resistance of the heating module in the glow plug.

As I mentioned in my previous posting, after the harness was removed from one of the suspect glow plugs (A) there is no connection to ground in the harness for that lead and (B) testing between the tip of the glow plug (where the wiring harness was removed) is shorted to ground.

Reviewing the MB service manual it references the relay having a built in short detection circuit that automatically disables +12v when a short is detected. Based on that it doesn't seem unreasonable that a fuse was not blown.