the Glow Plugs in your car should be wired in series, if its the original configuration with the waffle wire (touch one when starting and you'll think it can toaast waffles on them!)
Each GP in a series configuration takes only about 2.2 volts to heat properly. If you grounded the second plug from the rear of the engine you put a full 12 Volts across the rear plug and it most likely opened up from excessive current.
Do not use a spark test any more. Use a small incandescent light, or even a headlight if that is all you have, the best thnig would be a volt meter of course, as it doesn't draw any current.
Using an Ohm meter you can check GP's with everything OFF and the battery disconnected. I'll bet your in need of one plug. The old series connected plugs last a long time unless shorted by testing or something.
You may be able to start the engine by spraying a little WD40 into the air intake and cranking her, unless its really cold there.

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!