The buzzer is a legal requirement, I think the thing that may be confusing you, is the lack of mention on some witter instructions of the need to cut the original feed wires to the indicators, the buzzer is then inserted in between the original feed and the indicator lamp/s, 1 of the other wires on the buzzer goes to earth, the remaining 2 go to the trailer socket.
Look on
www.towingandtrailers.co.uk on here is a excellent, CLEAR diagram under 12N electrics.
The normal towing plug on cars is called 12N(for Normal), and is coloured black, there is an extra socket some have/may need that powers such things as caravan battery charging, fridge etc, called 12S(for supplementary), not needed though unless you have caravan really, so just don't fit one.
If your car has lamp failure warning ?, you will need a 7 way bypass box, if not you won't, even if you do have car lamp failure warning this can be left off it will just light up when trailer connected that's all.
Pm if you get any more stuck and I'll dig the neighbours mk2 focus out so I can tell you step by step.
tabetha