All GPS sat-nav's that I know of, installed into a motor vehicle require a hook-up to the vehicles speed sensor in order to function their duty. In this case, in the 124's, there is a module called a "Hall Effect" sensor. It works in conjuction with the cruise control (speed sensor) and also the climate control.

If the Hall sensor is not functioning, nor will your cruise control, or anything else it connects.

The taillight stop lights have a current sensing device that if you have a problem back there, say with anything less than Mercedes Benz taillight bulbs, or bulbs that meet exact same values, the cruise control will not work. So I have read. Some excerpts on this issue can be found in the "E-Class Owner's Bible"

Through the help of many who posted a resolve for my hunt in Audio and Tech Help, and diesel forum, thank you for your help.

The green/yellow striped wire is the speed sensor wire.