Depends on what car you have, most vehicles now have canbus wiring looms and various ecus, body control modules.
Soldering or scotch locking as most backstreet towbar fitters use can seriously damage the electrics or ecu on modern vehicles and can invalidate the cars warranty.
I would always go for a dedicated loom made by a reputable company if you can afford it.
Dedicated looms will normally work with the cars standard safety functions like trailer stability assist etc....
PF Jones wiring looms are very good quality and I was over the moon with the kit I bought from them for my Honda crv.
Ive been in the motor trade for 16 years now and I have seen some scary towbar wiring...........
Some people shouldn't be allowed to work on cars.