Intermittent Glow Plug Light

[SoonerFan]

I did a search here and somewhat educated myself on this subject but couldn't find this exact problem.
My son's 83 300SD with +353K miles has developed a problem I'm hoping to get some advice with. The glow plug light doesn't light and the engine won't start however sometimes without any explainable reason the light will come on and the darn thing will fire up great. I do hear something like a relay kicking off about 30 or 40 seconds after the key is turned but it still won't start. This problem cropped up the same day we had the oil & oil filter changed. I can't imagine what even a ham-handed oil change kid could do to cause this but it's the only thing I can think of. For some reason I'm suspecting something in the ignition switch??? I don't know why just a gut feeling, or could it be a relay?

Thanks in advance you can give me. I'm not much of a mechanic but nothing irkes me worse than paying a guy $75 to plug in a relay. If it does have to go to the shop I would still like to at least look like I know what I'm talking about.

[MB-Owner-in-ind]

Have you done some checking on the voltage at the plugs? You must check at least one plug for voltage. If you find voltage then unplug the relay and check each plug for resistance. Report back with findings.