It shouldn't take long if you can sort out which wires control what. When you checked fuse,did you confirm 12v in and out? Start at a couple of glow plugs to see if they are getting 12v when switch is in pre-heat position. Next is relay which is most likely where problem is. One of larger wires carry's 12v+ from battery to relay,find it with volt meter. Other larger wire carry's 12v to plugs when relay closes,if not and other has 12v,relay is bad. Smaller wires carry 12v+ from fuse to relay coil which closes contact between large wires. Another small wire run's from relay to light and a third ground's the coil. Try and trace the ground from relay to engine block or machine chasiss and make sure connection is clean and tight. With switch in preheat,if you connect and disconnect coil ground wire,you should be able to feel or hear relay close and open. If you hear/feel relay close but 12v doesn't go to plug wire,relay is bad.