Real basics is to start with voltmeter or test probe at glow plugs and work your way back along wiring till you find the voltage needed to operat/heat glow plugs, problem will probably be in the same general area..... As a guess there is probably a fuse involved or a relay operate by switch on dash.... Wiring diagram of particular tractor could be great help in tracing wiring..
I am kind of partial to this tool for tracing 'lectrical gremlins if no computer is involved...