Re: Glow plugs or not
The glow plugs, if working properly, draw about 90 amps.... enough to keep the alternator output voltage well below the normal 14.5 volts.
When you start the truck, note the position of the needle on your voltmeter.... let the truck idle for a couple minutes, and you should see the needle suddenly rise a fair amount after a minute or two. Or, if it's dark in the early morning, start the truck, turn on just the parking lights and let it idle for a couple minutes... you should see the lights suddenly get brighter when the glow plugs are shut off after a minute or two.
If you don't see this happening, the glow plug relay is the likely culprit, though there could be other wiring problems.