Of course you need to find the "right" wire.....not always the easiest thing in a hot itchy attic.......
Find enough slack in the line to make the connections........secure the box to the structure / remaining accessible........If it's metal use a grounding screw and connect the ground wires........
(same Tom?......a 4" square box 2 1/8" deep will handle 15-#14 "conductors".......size counts!...)