The socket is not a replaceable part. Its one big molded plastic piece that goes down into the headlight switch. The switch is not easy to disassemble and reassemble.
Unless we're talking about different things? Could you take a picture of this? What I am calling the socket is the bit of white plastic you see at the end of the metal post where the knob attaches:
As I said, there is no wire going to the back of the car, and there is no space available on the driver's side tail light plug to add a wire for the rear fog light. You'll have to run a new wire.
When was your 91 produced?
-J