Where exactly are the plugs in 1986 300SDL and how do I check the Ohm?

 

The glow plugs are located just underneath the intake manifold. You will need a multi-meter with an ohms position, put one probe to ground, and the other probe to the tip of the glow plug where the wire hooks up. this should be done after the engine has been sitting for at least 12 hours with that particular glow plug un-hooked from the rest of the system

 

It was the Pre-Glow Relay. The silver metal piece on top was melted and disconnected. I installed a new Relay and car started in 1 second as if it hadn't slept for a month!
So I blamed the Biodiesel fuel for nothing (unless biodiesel actually ate through glowplugs or something causing short)
Just to make sure I will use 50-50 Bio-regular diesel from now on and I will remove the tank-strainer after a couple of months biodiesel use to see if it's dirty.
Lesson learned: These diesels will not start without functioning glow-plugs even in hot 95 degree Arizona weather