Even if they were wired in parallel, one with very high resistance would make little difference. You can model it's resistance as approaching infinity, and it joins in with the other infinite resistance resistors in the circuit- the air between the glow plugs.

Having one GP out just makes that cylinder have trouble starting, which will cause some vibration, or in very cold weather affect the entire motor's ability to start.

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1992 300D 2.5T 249k - Parked with a bad transmission
1981 300SD 142k - Daily driver