s60,

You don't have to disconnect the battery to replace the wiring harness, although I don't think it would be a bad idea to do so. Make a diagram or take some digital picutres of the existing harness before removing. As I recall, the trickiest part was where the harness goes through the "heat shield" toward the rear of the engine compartment. I took my time, had a helper and it took about 1.5 hours to complete. A good portion of this time was spent drawing the diagram of all the various connections.

RM92400E