It might be easy to think "harness" when all you might need to do is repair/replace particular terminals which plug into the bulkhead connector. Replacing the individual terminals can be much easier than replacing a complete harness. But I also suspect that these terminals do not "short", but just have continuity issues instead, as they are all separated by the connectors that support them. I believe you can find OEM-level terminal kits online or from a few restoration parts vendors. Just make sure that what you get exactly matches what you now have (shape and configuration) rather than just re-packaged auto supply items!
Just curious, have you looked at the factory service manual wiring schematics? Usually, the wiring color code inside of the car is the same as the corresponding wire on the engine-side wiring.
Which circuit(s) are you having concerns about?
Just some observations,
CBODY67