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

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.