Just probe the cars wiring to determine the wires needed, after fitting bar solder the wires in, just leave off the indicators till last.
Now looking at instructions for the trailer beeper/indicator flasher, identify the indicator wires each side(will need to lengthen one no doubt), and take a wire feed from here to the beeper unit as per instructions supplied with it, then it will say take the (colour) wire from the beeper unit to pin no(whatever one) of the trailer socket.
The remaining wire from the beeper unit goes to earth, all this in effect does is put the beeper unit between the cars electrics and the tow socket.
It will be from the beeper unit two different colour wires which are hooked onto the indicator wiring(one each side), according to their colour, then there will need to be connected to the socket a YELLOW wire to pin position 1, this if for the left indicator, , it will also have a GREEN wire this is the right indicator from the beeper and goes to pin 4 of the socket.
Download the socket piccie to assist, if still stuck, pm me the wire colour designation from the beeper unit, and I'll draw it out and email you the diagram.
tabetha