I have been having trouble getting the SD started in the morning over the last couple of days. Yesterday I noticed the glow plug light wasn't coming on all the time and decided to replace them. This morning the car wouldn't start so my dad gave me a ride to work and told me he would look at it.
Well even though with a meter all the glow plugs tested ok he found something very interesting. When you stick a good glow plug on a battery the tip will glow red in a couple of seconds. Well he stuck all 5 of the old glow plugs on a battery and only ONE glowed red. The other 4 only got a little warm.
So the light was coming on when only one glow plug was actually glowing!
The point is if you are having trouble starting just spin a new set of glow plugs in. Now the car starts without much trouble. The $50 that a new set cost was worth every penny, I need the car to start.