The Data Cable LED indicates when there is sufficient 12V power and ground being supplied to the Data Cable.
OBD-I - The vehicle may be providing insufficient B+ voltage and/or ground at the OBD-I DLC. If required, the Lighter Power Cable plugged into the OBD-I adapter (see Connecting to OBD-I vehicles) may be providing insufficient B+ voltage and/or ground. The fuse in the Data Cable may also have failed.
OBD-II - There may be insufficient B+ voltage and/or ground at the OBD-II DLC. The fuse in the Data Cable may also have failed.