I'm not sure I understand the situation but I think you're saying you can plug in a block heater of some sort to start the tractor since the glow plugs don't work. Have you done any troubleshooting on the glow plug circuit aside from the fuse? Are the glow plugs themselves good? Depending on the tractor some have a relay which kicks in the glow plugs when energized. Others have a glow plug controller. Still others (older tractors) use a heating coil which glows and indicates glow plugs are getting power. It can also be as simple (or complex) as that portion of your ignition switch being burnt out. A bit more information would be a real help with remote troubleshooting.