This normally indicates a current problem, either too high or too low. (the light can flash after start up at VERY low temperatures, this is normal). The normal cause though of this problem is a glow plug going open circuit. If you disconnect the larger connector block from the glow plug control unit, you will have four wires in this connector which connect to the glow plugs in parrallel. Using a multimeter and a cold engine check the resistence down each wire to earth. The readings should all be equal and ideally from memory around 2 ohms. (Heat effects resistence so best to do it on a cold engine). If you have a resistence alot higher then this will need replacing, trace the wiring (the are all different colours) from the connector to the glowplug to locate which one needs replacing.