cleaning firewall

Got a 74 Duster and trying to clean up and eliminate all the uglyness of it all. There is a wire that comes from the firewall that is about 4 inches above the master cylinder. It has a rubber cone type deal coming from the firewall. Where does it go and what is it for? The inside of it goes to a cluster of wires i cant trace, help?

On the earlier cars, some had a grey wire similar in description that was for the tach. If it is grey, that could be a possibility...just a guess.

good luck.