79 300D W123- Neutral safety switch

I've replaced the glow plugs, strap fuse, glow plug relay and the ignition switch on my 79 300D. Intermittently, she doesn't start in PARK ( glow plug indicator light goes on , but doesn't start when the key is turned)-- If I shift to NEUTRAL, she starts fine. I pay attention to the warnings this" Panzerwagen" gives me , so I'm thinking that there's some glitch in the neutral safety switch.
Am I on the right track? Please excuse potentially incorrect terminology- I'm no expert, which id why I subscribe to this helpful forum.
Thanks in advance for your help.

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