one easy test us hillbillies use to test glow plugs is to hookyour test light to the positive side of the battery. unhook the wires from the glow plugs and ground the test light probe to the glow plug terminal. if the test light lights then you still have resistance and tje plug is working. if the light doesnt light then the plug is burned out. its not rocket science but it still works in arkansas.
hillbilly