In with the massively more complicated.
We stripped the old dash/bulkhead/engine-bay wiring harness out of the Impreza ready for the Liberty harness to go in. Notice the elegant simplicity of the Impreza wiring vs the mess from the Liberty.
Everything falls vaguely in place though and we should be able to squeeze it all behind the heater box. We'll need some new brackets for things like the fuse box near the drivers door and of course a heap of cable ties to clean it up.
Subaru miracles from this installment include: All of the holes in the bulkhead are in the right place and the Liberty bungs* fit (again, close enough for brute force), even to the point where an extra hole existed where needed on the passenger side for a branch of the Liberty harness that doesn't exist in the Impreza harness.
* We had a lot of fun saying 'bung' and 'bung-hole' during this process.
Not seen in pictures: The harness in and around the engine-bay fits pretty well - headlights, radiator fans, ABS and engine connections shouldn't be an issue - although side indicators, fuse box and gearbox connectors will need a little work.