As for the OBC - you cannot just pull out the 6 button OBC and insert the 14 button OBC. You can only do it if you change out some of the other minor harnesses that will work with the 13 button OBC. There is a tech thread on here that shows the complete conversion process. It's easy to do once you have all of the correct swap parts, just a little time consuming.
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Back to your original issue at hand...
The only other way to test such harnesses would be to check continuity at each harness plug by probing the metal inserts (ie: the connectors that get plugged into each headlight, etc). If you have continuity, then the harness should be good. If you have a break in the harness (at any point within the harness), then the continuiyt signal will be either weak or no return signal at all.
I would still visually inspect the main group of harnesses that control all lights and the light switch unit too. You may have to go as far as unwrapping the factory conduit or tape that's on the harnesses, as there could be a break under that wrap that you would not be able to see otherwise. It's a PIA process, but I think this will be the only way you will be able to fix the issue (and possibly prevent the situation from becoming worse).
Check the connectors as well, sometimes you may find a break or weak wire as it goes into the connector.
