MK2 Golf/Jetta Haynes manual has the diagrams. You probably want the MK1 Golf Haynes as well. Then you can combine the 2 systems easier. This is how I did the car that I built up.

Essentially you have the loom that runs the engine, this has a battery feed, ignition live and a ground. Then there's the loom that interfaces with the instruments, oil pressure lights, alternator light, starter motor etc.

Gavin