I've not seen your tractor but glow plugs are similar to spark plugs. There will be a wire running to each. You disconnect the wire then use a socket to remove the glow plug. You may want to check to see if they're even getting voltage, though. The coil on the dash doesn't usually glow very brightly, it's usually about three turns of nichrome wire to drop the twelve volts from the battery to the 10.5 volts for the glow plugs. The wire heats and the connections oxidize, then nothing passes. The resistance should be readable but it's pretty low - an ohm or two. If you get a really high resistance across the coil you're getting nothing to your glow plugs. Of course glow plugs can fail open or shorted, too. I think the part you're looking at with the bolts may be the injector.