You should be able to pull apart the bullet connector on the turn signal assembly, connector is somewhere under rear fender, then wires and indicator shaft can be readily installed through new rack and frame.
The seat removal can be done a variety of ways. A 10mm socket on 3 inch or so 1/4"extension and 1/4 drive ratchet works. I use a small swivel head 1/4" rachet and a 10mm socket. Many other favorite tricks including quick release fasteners are used by us.