My 1720 has an idiot light to show I have the glow plugs on. It times out after 4 seconds, I believe the manual said, but in real cold weather you can leave it on longer.
Now, my BiL's older Kubota tractor has a little coil spring deal that you can see through a plate in the dash. As you hold the glow plugs on, that coil starts to glow red, and you know how long/ hot the glow plugs would be. It looks like a toaster or electric oven coil, sorta.
From your description, sounds like you have that glowing coil deal, & not an idiot light? I would suspect the idiot light don't matter if it don't work, and that the glowing coil of wire does matter.
--->Paul