The Xj uses a few harnesses that plug into the over all harness to do things like rear heated glass (defroster), power mirrors/windows and over head consol option just to name a few. Some things are already there because it is cheaper to include them in a normal build in piece parts as in the tailgates, some are not, like doors (power versus non-powered). The year of the XJ also has a bit to play in what's there and what's added.
Most likely you have the connectors hiding in the kick panels for the power options but the rest of the big harness is not.
Be prepared to pull a big hunk of what ever XJ you get the sub-harness from apart, there are relays under the passenger side kick panel, wires going down both lower door jambs to the back doors and one to the tailgate if equipped, plus the power mirror switch has wires run under the drivers seat if I remember right.
And all the motors to go full power.
Good luck and take your time, and a camera phone if you need tips to rewire into your rig.
Andy