philac, Have you ever taken a glow plug (out) and connected it to a battery and see how long it takes to get RED hot?
A good GP will take a few seconds, maybe 5-6 seconds.
Have you seen a GP that has gotten so hot the end swells? Have you seen a GP that swells to the point it does not come out of the threaded hole it's mounted in?
I'm just trying to get you to think about why you need the GP's on so much, and what can be the results.
I can't see a problem with the wiring of a separate switch, but I'd try to figure out why... is it low compression, bad glow plugs in the first place, or some other reason the engine is not starting.
I'm pretty sure on your tractor you turn the keyed switch one position clockwise, then turn the start switch counterclockwise and hold for your decided time for GP activation.