You should be able to re-use the engine loom from the car - just strip out everything engine related, ecu and its wiring. Normally the engine side of the car is pretty self contained with a connection into the cars ignition system for power and starting.
If you have a look at my wiring diagram thread you should see a basic circuit diagram I have created. It covers all the lights, horn, rad fan etc and also hasa circuit for the megajolt I am using. You could use this diagram and just change the megajolt circuit to feed you ECU from the relay. The diagram includes a changeover relay to only the fog lights to only work with dip or main beam.
I am working through my wiring at the moment circuit by circuit - just start connecting the wires up one by one, leaving the main power feeds, hook the battery up to that independent circuit and test, then when all complete add the main power feeds. I must admit I'm finding the wiring surprisingly relaxing and enjoyable - I was like a big kid last night turning the lights on, then off, on, off, on , off for 10 minutes
