my towbar (from Towequipe) just came with a pre-wired socket and scotch-loks

.
The colour codes used are.......
Vehicle............... Trailer cable
Red/Blu............... Brown & Black (there seems to be only one source) ........ Side/tail lights.
Pink.................... Red ....................................................................... Stop lights
Green/Blk ............ Green .................................................................... LH Indicator*
Green/Yel ........... Yellow .................................................................... RH Indicator*
Black ................. White ..................................................................... Earth
Blu/Pink .............. Blue ....................................................................... Rear Fog
Blu/Whi ............................................................................................. Reversing lights
* - may need to go through a warning relay (mine did).
If I was going to do it again I would use a Molex 3901-2120 with 3900-0039 contacts and the mating 3901-2121 with 3900-0041 contacts. that way, using 4 links, it would be completely plugable. I had to solder the indicator buzzer relay to the tow bar cable.