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It absolutely does. A wire inside of a harness is a closed system. You have water tight sealing on the end of the connector on both ends of the cable. As soon as you open the sheath and expose the bare copper to atmosphere, corrosion starts. You cannot splice an automotive harness and keep the same characteristics as when it came from the factory.

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Most automotive wiring harnesses have crimped connectors, these leave the ends of the wires open to the atmosphere. Connectors used under the hood tend to have seals these effectively seal the wires as well. Wiring used inside the cabin (or anywhere not exposed to the elements) generally don't have any seals to save cost. As long as you're alright at soldering and heat-shrink the wires they'll be fine, this really isn't much of a problem.

None of this is true. Wiring harnesses aren't water tight, primarily to prevent corrosion. If you have a totally water tight harness, and water some how gets in, it has no way to dry and will cause corrosion. You absolutely can splice a harness and have it have the same characteristics, I do it every day.

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...

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