Problems starting 1981 300D

[openshift]

OK, just as a disclaimer, I do not have much experience with electrical problems. I have spent the last two weekends going between research in this forum and testing the suggestions on my car. The advice I have found has been great and has really shown me how more I have to learn.

My problem is this: when I try to start my car, I turn the key to let the plugs glow and once the light goes off I turn the key fully. When I do this, I get a loud click, but the starter does not seem to engage. I just replaced the starter with the help of a neighbor and also replaced the battery as well. I have checked all the connections from the battery to the grounds, to the starter and from the ignition. I have jumped the connections in the terminal block (terrifying) and the starter sounds as though it is working correctly. I have not been able to locate the NSS on my own yet and will be looking into that shortly. A friend was able to get my car to start in neutral, but I have not been able to replicate that. Having done all of the above, what might I have missed?

Again, I am very new to this so please bear with me. I know that this is a fairly well covered topic and I apologize for adding another thread to the pile. Thanks in advance for your help.