OK, we have determined that the glow plugs are fine and the glow plug relay is fine.

So, the issue with slow cranking speed needs to be addressed with this engine. How old is the battery? If it is more than four years old, replace it with a decent battery. Auto Zone makes a battery with 850 CCA for $65.00. It's worth the peace of mind. If it is less than four years old, take it out and have it checked under load to determine its output. I appears that it is not providing the CCA that you need to get a diesel going in very cold weather.

What oil are you using? Engines that are "difficult" to start should be using synthetic oil in the winter months. I use Rotella synthetic, however, Mobil 1 is generally considered a better oil, but it costs more.

How long do you leave the key in the #2 position before cranking the engine? The glow plug light will go out in ten seconds or so. In very cold weather, you should be waiting 35 seconds before turning the key to "start".

What is the conditon of the battery cables? Make sure that the cable to the battery connections are clean and solid.

Keep in mind that a diesel that does not start in the first 10 seconds loads itself up with fuel, thereby making subsequent attempts to start it all that much more difficult. When you go to start it, do not give it more than 1/4 pedal. Excessive pedal will inject more fuel than the engine needs to start and all that fuel must be heated up to combustion temperture or it will not fire.