First quick and dirty is to feel the "hangers". They should get hot as they're really resistors just like the glow plugs. Then I work backwards from the ground at front cylinder, checking the dead system for shorts to ground with an ohmmeter. Make sure the ground is good and clean. Three places to check at a glow plug: plug tip (the end you see now) and both sides of the ceramic. The system can short from carbon or a cracked ceramic, then blow (open) the plug upstream of the short (from too much current). Carbon means you need to ream out the hole. A blown plug without a short means you probably have a drippy injector (water or dirt in fuel). Treat those glow plugs as a red flag that points you to the real problem. The salt shaker is a "glow plug": you'd better get less than battery voltage downstream of the salt shaker. 9v is a bit low: check and clean the battery connectors and the starter ground strap.

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daBenz - 1970 220D