Hi David...
Go to File->Options->Assembly and check to make sure you do not have restrictions on so-called "out of scope" references.
There's an option in assembly mode to prevent you from making inadvertent references to geometry not in the current model. Those references would be called 'external references'. When cabling mode is active, you're actually down inside of the cable/harness part. Your intent is to be able to select surfaces or other references from other components in your assembly. This is "out of scope" for the harness part and represents external references.
I believe if you go to File->Options->Assembly and change the top option Components Permitted for External Reference to All, this will solve your problem. You may have to get out of the model and get back in after you change the setting. If you read the other options in that area of the Assembly options menu, you'll see that external references are, by default, painted a dark gray color to indicate they are not eligible for selection. This is dependent upon your settings for external references of course. Pick the graphic below to view in a larger window.
Please try that out and let us know if it solves your problem.
Thanks!
-Brian