Haynes' (Mercedes Benz Diesel 123 Series) has a diagram for your year/model. It shows these connections to the starter:
Black- Battery +
(2)Red - Alternator
(1)Red - Ignition Key
(1)Violet - Air Coditioner Relay & Glow Plug Relay

It sound like there may have been problems with the ignition switch and a separate "rig" was set up for starting and glow plug. (I hate finding rigged crap like this on a car. It makes me wonder what other maintenence isn't up to par.) Hope it's easily sorted out.

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Sam

84 300SD 350K+ miles ( Blue Belle )