We've tried starting by bring positive directly to the first glow plug. (This is a 1976 300D, so the Glow plugs are in Series.) And the ground after the last glow plug is also working.

I agree that I must have messed up something, and since we have double checked that fuel is getting to the injectors, and that the glow plugs are on, I stretching to figure out what else I might have done wrong.