Well thats part of the problem with 20 year old wires adding large sound systems and increasing amp usage.
Personaly I would try and speperate the lot into positive, neutral and grounds then make sure they are water tight most drain holes are in the chassis legs which don't help, plus water in the rad area from forced air water on the front of the car! IMO you can do it properly but most people won't due to time and effort involved. I have seen bays that have false sides back and sides so they create cavities that are water tight (EXPENSIVE) and those boys are high end as they really don't want any wires to be seen and also delete car componets and relocate for minimalistic bays. With enough money you can do anything well
