I have not checked a wiring diagram, but maybe someone knows ...

If the contacts in the ignition switch go bad, would that cause the glow plugs
to not work ?

I have removed and cleaned the contacts before on the 300D ignition switch.
I imagine repeated use would eventually cause failure.

I checked resistance on all 5 glow plugs from the connector at harness.
Number one was bad so I replaced it.

I checked voltage at the relay on both sides of fuse and it is 12 volts.

I next checked the voltage at each of the five posts on the relay
for 12v when key on, this worked until the relay clicked off.

When in the car I get no to very short glow plug light
(cold morning at 30 - 40 degress F).

I noticed that if I wiggle the key around and go from off to on several
times in a row I can get the glow plug light to come on.

It makes me think ignition switch, if so, how common is ignition switch
failure ? Car is the 300SD 1983 with 263,000.

Do most of you take apart and clean contacts, or just replace ?

__________________
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

1984 300d Turbo 146k
Order A New Ign. Sw. **************.com $37,95 Guide, Ign Sw, Tool To R&r

For all practical purposes, the glow plug light is unrelated to the actual operation of the glow plug timer/relay. If your glow plug relay is remaining closed for the appropriate amount of time, your problem is probably not the ignition switch.

How long does your relay remain closed?

Relay is staying active for a good long time.
I did not put a stop watch to it, but felt I was easily
at 20 seconds (only expected 10).

I was able to move my volt meter probe from post to post 3 or fours times
around before it gave a noticeable click and shut off.

At first I assumed it was the bulb on the dash for GP that went
bad and simply waited an appropriate amount of time to heat the GP's.

Except that the car was hard to start, which made me think the GP's
really were not getting hot.

Since then I know the bulb is okay.

I disconnected the harness at relay, took it apart and cleaned it and
crimped the connectors a little to make good contact.

Oddly when I fiddle with the relay connectors or GP's the dash light
seems to come on as expected, but only once or twice.

This weekend I will pull the ignition switch and study the contacts
to see how corroded they are.

__________________
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

That would seem to suggest a problem with the glow plugs and/or wiring. Have you measured the resistance of each plug/wire? It's possible that you have an intermittent problem downstream from the relay and the fault detection circuitry (which controls the light) is operating as it should.

If the relay is powering the plugs for around 30 seconds, it sounds like the timer is operating normally.