I replaced my battery on my 1994 968 Cab. and failed to label the cables after taking them off. Everything is pretty straightforward with the reds however I have a black cable coming out of the firewall that I don't know if it is positive or negative. Because the cable is black it leads me to believe its negative but I seem to remember that there were only 2 negative cables attaching to the battery and this one would be a third. Does anyone have an idea what it can be and if I should hook it up to the negative side or positive? Would any damage be done if I connected it to the wrong side? It's a good size cable that is wrapped together with another wire that looks like it goes to the cruise control.

 

Put one lead of an DVM, or ohm meter, on the wire and the other on the chassis ground.
If it reads close to "0" (zero) ohms then it should be a ground wire.
If it reads higher than a few ohms, it's not a ground wire and you will need to trace it out.

Pictures the next time! Lots of pictures!