If your "indicator" is a coil of nichrome wire like many older tractors you may NOT see it glow. If it burns out or opens in any way though your glow plugs won't work at all. They're wired in series with it. Glow plugs run at 10.5 volts. That coil of wire drops the extra 1.5 volts.
With the Key in the start position, the glow indicator is bypassed and the GP's receive power.
This feature an cause confusion when troubleshooting the GP's
Study the two schematics and you will see how the GP indicator is bypassed in the Key start position.
In pre heat the power flows from Terminal #30 to terminal #19, then throught the GP indicator and finally out to the GP's via terminal #17.
In the Start position power flows from terminal #30 to terminal #17 (bypassing the Indicator) and out to the GP's.
Dave