Help installing Air/fuel gauge

Im running a 93 cherokee 4.0L and i'm having some trouble finding out what color signal wire is that I tap off coming from the ECM to the 02 sensor. I looked all through my chiltons with no luck. If theres any guru's out there that can point me to the correct one that would be greatly appreciated!

Black maybe? I'm not 100% sure.

The white ones are for the heating element and the grey is ground I think.