Ignition Tumbler or some relay?

[nyko46]

My 1984 300 SD Mercedes Benz is giving me a problem that I never had before and I am confused if it can be from ignition tumbler or something else.
Here are the symptoms: when I go to start my car after waiting for glow plugs getting ready to start the starter does not go on even turning the key couple of the times. Then my only option to get the car running is to open the hood and manually close contact directly on the connector where the wires to the solenoid are connected. Doing this starter will engage and my engine will start.
One thing call my attention when it happen: this does not happen every time when I go to start my car. It can go for days without having this problem and sometimes it can go many times with this problem. Turning the tumbler to the on position all electric components of the car will work as supposed to work except car starter.
I do not know if there are some mini relay between tumbler and starter that can go bad and cause this problem. I know that all electric contacts on the connector near the fender are all OK.
Please give me your help to get this issue resolved.