I never owned one of those rocket ships. Although there was one owner that set a speed record by driving it off a cliff.

The salt shaker may have a glow plug in the series as an indicator in it. If it is burnt out it could be your issue. Also there should be a strip fuse perhaps on the firewall. Anyways easy to find the issue.

Feeding the current through the headlight switch is not likely as the system current is so high. If you live in a temperate part of the country. Keep something in the trunk to start on if a glow plug fails. They are like an old series Christmas tree light string. One burns out then they all do not work. Never test one of those glow plugs using twelve volts.