We have discussed this multiple times.
In a stock system, GM designed the system to feed 24v to the resistor pack and then to the plugs.
When the system is functioning properly, the resistance of the the resistor pack and ALL of the glowplugs drops the voltage to 12v
The plugs are 12v versions.
If one plug burns out, then the the total resistance drops and the voltage increases to the rest of the plugs. This causes more to burn out and the voltage to continue to increase until the last "good" plug is receiving the full 24v and has a very painfully death.
This is why some of us recommend removing and bypassing the resistor pack and residing the GP system for 12v