Thank you Yak. Following the directions was easy to do and very helpful.

And my glow plug light now works as expected.

The 1980 300D that I have has two temperature sending units in the head.
One for the pre-glow system, one for the temp guage.
Later models went to a single temp sender.

The end result is I ran two new wires from the sensors to the glow plug relay.
Wires 6 and 8 on the relay in the harness from glow plugs.

The connectors on the sending unit wires were both down to one strand of copper each. Just poor, old , brittle, hard, wiring.

Using standard bullet connectors (thank goodness they are the same size)
I was able to run two pieces of wire from each sender to the realy system.

Glow plug light works like I remember. Great news.

This car is to be passed on to yet another young driver in my family
and I wanted to get everything just right as the beginning driver learns to drive a diesel .

I have made this a right of passage and have taught 6 beginning drivers in this car,
now on to the seventh and last driver.

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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