From your question about sockets you obviously are not up to doing the job yourself as connecting the towbar wiring to the wrong car wiring could blow circuits within the cars electronics. If you're only ever going to tow a simple trailer or horse box a 7 pin black coloured socket is sufficient for supplying the tail and brake lights. If a caravan is being towed you need the additional white socket to suply the caravan fridge/battery while towing, this wiring has relays fitted within the car to prevent reverse current being drawn from the caravan battery to the car battery. The 13 pin socket is a new (German) idea which amalgamates all the connections into one socket. All caravans since about 2010 will have them, older ones wont. Fitting a towbar needs to be done by a qualified fitter nowadays, its not the mechanical side of the job, its the electronics. At least, if a fitter gets it wrong, his firm has to fix it at their cost