Check voltage first, then check the glow plugs first. If they are fine and you are having these issues, I'd check the glow plug thermostat, then the control box.
The "Wait to start" light is an indication if the box things the glow plugs are working. Depending on the box, some just energize always, some based on temp, and some will test if the glow plugs are any good flash the wait-to-start if they're bad.
To check the voltage, (make sure the battery meter is right), when you turn the key to the 'run' position, does the meter drop into the Red or just at the Red/yellow junction? Or does it sit in the yellow? In my experience if it sits in the yellow, the glow plugs probably aren't on.
Next you can check the glow plugs, but you need a DC compatible AMP meter, then you can see if power is getting to them (and being used).
Next step is probably to remove a glow plug and check it. If you do go down to this, make sure you buy REAL glow plugs, there are a LOT of counterfits out there. The way to tell if it's real, the glow plug head will be 3/8" hex, NOT 10mm. If it's 10mm it's counterfit! Also the conenctor on the head of the glow plug will be the military bullet connector, not a flat spade connector.