A whole bunch of folks answered this question for you in the main diesel forum. The forum moderator even posted a cutaway drawing of where the glow plug sits....
1980 w123 glow plug wiring
The issue here is that the WVO has to be heated BEFORE it reaches the injection pump and maximum heat has to be achieved before it reaches the injector.
Otherwise you are pushing thick sludge through a precision instrument (the pump) that is designed to push diesel fuel -- and doing the equivalent of sucking bacon fat through a soft drink straw in passing it through the injector. Trying to add heat by means of the glow plug AFTER all that has occurred is next to useless.
ADDITION: Okay now I think I see where you may be going here... I came across your earlier posting about dumping in the WVO before you installed a kit, etc. Are you simply trying to get the car started? If so you should just ask about that.