Can't say where but might suggest that activating it you should hear a solid click or clack as being a high current device it would announce its presence.
I own a E-230 and there is no relay, it works right off of the 12v going to key switch, I have the wiring schematic, and it does not show a relay, but its not a E-280 so not sure about 280, but my operators guide does cover both.
Your tractor may not have a relay. Many Japanese tractors of that vintage used a dropping resistor to drop the twelve volt power to the 10.5 volts the glow plugs use. It took the form of a kind of glowing indicator, usually a round thing with holes, on the dash. Inside is a short coil of nichrome wire which gets red hot when activated though you may or may not see this action. Over time the terminals tend to corrode or oxidize and you may no longer have adequate current to power your glow plugs.