I used 18 gauge since it's only an LED. THe window locks are already lit up. It is the lock switches that some people have that are not.
There is no splicing of the wires, your going to just stick the tinned end of the wire into the back of the socket (plug) it should fit nice and snug with no need to cut wires. You need 1 ground wire and 1 hot wire for a total of 2 wires for each door.