Ultimately you're going to need reliable wiring between all the power sources, fuses, switches, relays and the powered devices. If the existing wiring is unreliable then you're going to have to replace it, and whatever you replace it with will need to be properly made and installed e.g. sealed and with strain relief where necessary.
That doesn't mean you need to go to a digital control system. The simplest approach i.e. using the least number of wires and connectors is to just wire everything up directly as the original solution does i.e. just reloom it. Granted that in theory you can reduce the quantity of long wiring runs by using a common supply rail and a digital control bus, but that increases the number of wires and connections and points of failure and complexity dramatically. Unless you have some ulterior motive you haven't told us about, it doesn't seem like a sensible approach to take.