Maybe I can help answer this question having installed 2.2, 2.5, 3.3 and 2.0 turbos into Vanagons.
The wires that need to be integrated into the Main wiring harness are for the following:
Check engine light
Fan relay 1
Fan relay 2
Fuel tank pressure sender
Fuel tank valves (2 or 3)
Transmission (if automatic)
Other than these wires, the connections needed from the new harness are connectors that go to the engine, ECU, O2 sensors, diagnostic connectors, fuel pump controller, and the air flow sensor.
I may have missed some connection in this list since I'm working from memory but this gives you a starting place. Simple eh.