Hi Matt,
I built my Ulster Special (Rod yates body) from scratch and did my own wiring, no loom as such. I ran the battery wire to the starter down one side of the chassis(12V rear mounted battery)in some protective sleeve.
All the wiring to the rear (only 4 wires but I included two spares) ran down the other side of the chassis in plastic flexible sheathing that came up through the body with a loop to a junction box in the tail. Then some nice looking wire wound cable to the lights (from seven workshop) I used modern metal flexi trunking and wire for the lights at the front but it looks quite original.
Someone posted a good link to wiring, I also use this more up to date diagram which worked better for my brain.
For the cut out I used a 12v one that is in a correct-looking case, I just disabled the regulator part that came with it.
Relay board is on a piece of ally under the dash with loops so I can drop it down for access.
I mounted a metal clad resistor under the bonnet in a pre-war junction box for the dynamo summer/winter setting.
I can send pics and info on the materials/suppliers I used if you are interested. Or you can have a look if you are near Camberley!
Paul.
