re wire question

As long as they only need 12 volts it would be possible, however, wiring the rear window glass direct might not be a good idea, when you use the door hendle the window drops slightly to protect the glass, then goes back up when the door is shut, to seal the door.

The washers and heated rear window would only need a switch, relay and fuse, obviously seperate one of each for each circuit.

The hard part would be making it look half tidy, that would mean lifting carpets etc to run the wires.