On my 1980 300D ( a little different - no-turbo) the rearward one
is the temp sensor for the dash instrument (test it by unplugging),
the forward one is for the glow plug temp sensor.

THis is according to the factory service manual wiring diagram.


I think it was green wire for temp instrument guage.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250