You have a glowplug light
You already have a glowplug indicator light on the dash that comes on when the key is in the number two position. In most cases it's preset to supply voltage to the glowplugs for 45 seconds. In a lot of cases it goes out when the engine cylinder is heated enough for the engine to start. In colder climates I generally wait until my seatbelt buzzer goes off which is approx. ten seconds after the glow plug light goes off. Sounds silly I know, but it actually allows the cylinder to warm up that much more and improves starting in cold weather.
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'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan