Glow plug light & plugs only come on when cold?

[Panzer_Man]

83 300D turbo in Florida

The last few months the Panzer's glow plug light sometimes comes on, sometimes it doesn't. When it doesn't, the plugs obviously arent' operating because it takes about 8 seconds to crank the car. I put a used, supposedly good glow plug relay in it and it did not resolve the problem. I then had my mechanic replace the glow plugs; no change in symptoms.

Interestingly, when it is really cold, both outside and the engine, the light seems to thankfully always come on and the car cranks right up. But if it is mild outside the light doesn't always come on leading to the long crank (and smoke).

Is there a sensor that says "if engine warm do not turn on glowplugs"? If there is, that may be the problem.

Also the relay may not be good as it is used. Also note that the relay that was in was a new part put in about 2.5 years ago. So perhaps I just need a new relay. (?)

Ideas?

Thanks in advance,
Matt