Plastic housing around wiring on driver's side
 

I'm finally getting around to installing my door speakers and there's not a ton of room for wiring underneath the floor skirt on the driver's side. The existing wiring is running through a plastic housing that's covered in electrical tape. Any reason I couldn't cut the top of this housing off and run my wiring on top of the existing harness?

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fixed pic

If you mean that you want to cut the wrapping off of the Nissan wire harness and merge your speaker wires together with the Nissan wire harness, then go ahead and do it. The wrapping is mainly for manufacturing and assembly line convenience.

Removing the wrap would allow you to mold the 2 harnesses together and get all the wires to fit in under the plastic cover. I would get some zip ties and re-bundle all the wires together when you are done so that none of the loose, individual wires will work their way out from under the plastic cover.

I was referring to the plastic casing below. It's covered in electrical tape and there's an existing harness that runs inside of it. It's only underneath the driver's kick panel so I wasn't sure if it served some purpose other than protecting the harness. The harness that runs through it is also wrapped.

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