Just to be sure, touch a probe to the electrode of the #2 plug and check the resistance. Discount a bad wire before you remove the manifold. Of course, if it's a bad wire you'll have to remove the manifold to replace it :/

You might have the right tools and 5 knuckles on each finger to get to the plug without pulling the manifold. Certainly try first. If the manifold has to come off, consider replacing all the glow plugs. I've heard good things about Bosch, okay things about Beru, not so good things about Autolite.

Sixto
87 300D