Cheap afterglow - disconnect the purple wire in the small 4-pin connector at the glow relay. This will cut the starter signal that tells the relay to stop glowing and you'll get the full glow count every time.

OE afterglow - get the afterglow relay from a 90-93 300D 2.5, add an afterglow temp sensor to the fifth pin in the expanded small 5-pin connector at the glow relay. One caveat is I don't know if the relay will notice a missing glow plug and use the dash glow light to indicate a fault. But the glow feature will work.

Sixto
87 300D