I make my own harnesses. I use positive and negative post that pass through a section of hose surrounded by a gromet into the interior and run to distribution fuse blocks usually with a cutoff switch in between leaving no chance of battery drain.
This allows for a system that can be added to without rebuilding the entire harness. I use blue sea systems. They are a quality product at a fair price.
This is what I have used in the past and will be using on my current build here soon. I use cutoff switches from the same company.
Amazon.com : Blue Sea Systems Fuse Block ST Blade Dual 12 with Ground and Cover, 5032 : Sports & Outdoors
www.amazon.com
It is a split blade system letting you have 6 constant and 6 key on.
You want more fuses simply buy a 2nd or 3rd box. I use 0 to 4 gauge wire to feed these boxes depending on how many and the fuses all feed relays. It makes for a very clean setup.
As far as wire goes I use 4 gauge (pos/neg)to feed relay distribution blocks (10 gauge to the relays) at the front, rear, dash, and middle.
This not only gives you full voltage at the sources it lets you run 14 gauge with a majority of the rest of the system keeping lower cost switches(factory rated) and wiring.
Now as far as switches and plugs. I use factory stuff. But I pull the pin terminals and then reinsert them so they are tight. I then use 4" zip ties to hold the say 3 wires together. This prevents one wire being pulled if the bare harness is grabbed/snagged.
I shield my wiring inside coolant or low pressure fuel hose. You carefully cut down the length then push the wire inside before using some 4 or 6" zip ties. These keep your wiring safe from heat, chemicals, uv, and can be cleaned much easier then plastic looms. If you need to replace a section that say laid on a header then it is a trip to your local parts store to buy it by the foot.
Now building your own will cost more. But you will know it is built right and will be able to trace issues down easier. By using distribution blocks that have lables you can see that say the taillight fuse is blown. That is the orange wire with black stripes(marker) every inch. Go back to the relay and see that you accidently ripped it out of the relay when you threw that tire into the trunk and it grounded( proper routing and a zip tie would have prevented.
Plug the orange wire in and then replace the fuse then your good to go.
Vs I have no taillights. Replace the fuse then wonder what color my taillight wire (no relay)are with no way to check but to strip my harness and hope I dont pull any other circuits that may be T'd within that harness.