I am going to give you a bit of a generic answer believing your Ford is a Japanese tractor in disguise.
If the glow indicator is a glowing piece of coiled wire, its real purpose is to lower the voltage applied to the glow plugs to around 10.5 from the battery's 12 volts.
The key switch plays an important role in glow plug operation. Never use an off the shelf switch.
When the key is turned to the pre heat or heat GP position power flows through the key switch, through the glow indicator to the actual glow plugs.
When the key is turned to START, the key switch bypasses the glow indicator and sends power directly to the GP's. The GP's are powered while the engine is cranking.
The glow indicator is bypassed during cranking because the current draw of the starter is already lowering the voltage the GP's see.
Replace the burned out indicator. Work around tricks risk over heating glow plugs which will explode inside the cylinder leaving hard shards of ceramic which can cut into any metal destroying your engine.
If you cannot find a Ford glow indicator, likely a Kubota
B7100 will do. Measure the resistance of each glow plug to ground with the wiring removed. You should see around 1.0 ohms and if you do this is further proof the Kubota indicator will work.
glow indicator
complete indicator
Dave
M7040
