These might help a bit

http://eva2.homeip.net/W115/program/Engine/615/15-510.pdf

http://eva2.homeip.net/W115/program/Engine/615/15-505.pdf

I would run a min.6mm2 wire through the ballast resistor on the firewall from + term. to GP on cyl.4 and check function. This will say whether the short is somewhere upstream (pre-glow/starter switch). Then investigate. Measure the resistance and voltage as shown in the docs ,also A between the ballast resistor and GP #4.
Do not run battery current w/o the ballast resistor, it'll fry the GP's.

IIRC at starting (when you pull the knob to operate the starter) the salt shaker is bypassed to give the GP more current.