My car (1997 E300D) will not start sometimes. The starter engages properly but the glow plug light does not come on. I lose the voltage that engages the glow plug relay anytime that the glow plug light does not come on.

I can turn the key a few times and then the glow plug light will come on, I get voltage to engage the glow plug relay, and the car starts fine.

What would cause this intermittant failure of the glow plug light and simulantaneous loss of voltage required to pull the glow plug relay in. Is the ignition switch the only possible culprit?

thank you for ANY thoughts you may have.

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Neil
1997 E300D - naturally aspirated
280,000 miles (as of November 2015)