Most arcane source for an electrical draw I have read is one where minor coolant leak was dripping onto the alternator. Took the OP crazy time to figure out.
If your car sits outside, check your exposed wires for mice bites.

If you need a new glow plug relay PM me with the part no. so I can cross reference the one I have.

Since the 1982 300CD does not have the KLIMA module, that is one source of common electrical draws to strike out.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD