Pretty much all wiring repairs can be done on the vehicle. It can be difficult to install a harness in the exact original position in terms of how the harness is routed, clamped onto certain parts etc. If you were to remove it and try working on it, things are different lengths and need bends and twists in certain spots so you really need the vehicle there to go back and forth to match things up and cut everything to the right length.
Best to leave it in place address each circuit one at a time. What problems are you having?