one wire left 1972 300sel

[robertbrown29]

I just finished taking out my battery tray to clean underneath it and to clean the neg terminal ground and the bundle of ground wires attached to the fender.(passenger side)I am left with a wire I dont remember seeing when I began this process.It comes out of the firewall and has an 0 shaped connecter on it with a white tab just below the connector.the wire is black with a black cover.although the connector is the same as for the ground wires,I dont think this is a ground based on the color.there is a ground wire of equal length,but I attached it to the main ground.this wire can travel as far as the last fuse connector on the fire wall,but I cannot find a place for it.Hint:the car will not start now.I would be gratefull for any help.thanks