77 300D gloplug doesn't stay on

I have a 1977 300D Engine # 617912-12-008247. When I turn on the
ignition the gloplug light stays on only about 2 -3 seconds and goes
out. In the warmer weather and when the car has been running recently
there seems to be no problem. I was told that it might be the sensor on
the front left hand side of the block. I have tried to find a parts
manual expanded view on the web.
1. Might this be the cause of my problem? if so how do I test it? how
much for a replacement part? new? used?

2. Where might I get a parts manual? how much?

I'm doing this for my Dad who is 81, any help would be greatly
appreciated.

Carl