Here is the portion of the wiring diagram for my TC29DA that covers the glow plug circuit and some pages from the service manual that contain the glow plugs, fuses, and relays. It's not the exact same tractor, but what you are looking at must be the same or really close: the glow plug circuit is not that complicated. I have experience testing my glow plugs (ohm and voltage), so you could search my name for other related posts, including testing/replacing the timer relay and adding a push button to activate the glow plugs.