We are talking about two different things. Notice I did not say wire harnesses are water tight. Each individual wire inside the harness - the copper inside - is

We are talking about two different things. Notice I did not say wire harnesses are water tight. Each individual wire inside the harness - the copper inside - is sealed to the environment. You will get corrosion on copper if you expose it to atmosphere - exactly what happens when you splice it. By the way - I do development / testing of automotive electronics and am well aware of the specs / test procedure and design decisions of wire harnesses...