If the glow plugs are still installed, measure resistance to ground at the 6-pin connector into the relay. Each should be just less than 1-ohm. My guess is you'll find one grounded.

The glow plug terminal is insulated from the flats used to tighten the glow plug in place. If you bend the loop connector such that it touches the uninsulated section of the glow plug, you'll have a short circuit. A glow plug wire with poor insulation is also a candidate for a short circuit. Find the short circuit and fix it.

Sixto
87 300D