Supply request.

1.5mm bit small for brakes..

The smallest we can buy is 2mm in a 5 or 7 core, minimum 5-6mm is ideal for trailer brakes.

I think he is talking about 1.5mm squared?

sensible answer. IMO

Sorry not true.
Once you step into the world or real electrical cables, an great deal of things are available

12 core 1.5mm@ flex is reasonably common in theatrical lighting and can be purchased from any good electrical cable supplier.
I have nearly 200 meters of the stuff in my shed.

heavier multicore flexes can be had and are quite common in control cables for a variety of electrical items including cranes, lifts and the like.
2.5mm2 is far from uncommon

far less common are cables with a mix of cross sections.

3mm automotive cable is arround 1.1mm2,... 4mm automotive cable is arround 1.8mm2,...5mm automotive cable is arround 2.9mm2.

BUT
I'll go with what others have said...forget the 12 pin plugs, 7 pin plugs for the lights and other better plugs and seperate cables are a better bet for the auxiluaries.

cheers

Hate to tell you yes it is true, most people who want to use a 12 pin plug and socket and are asking for 12 core cable will want to use at least 2 cores for battery charging (-+) and maybe another core to power a fridge.

Hence why I said "nobody does a 12 core cable suitable for what you want it to do"..... unless you 12 core cable can defy physics and is capable of providing feed to charge a battery and run a fridge (3 way)

It's called specifying the right product for the job.

I don't see the point in sending someone on a bum steer getting his hopes up and then telling him it's not suitable for his needs.

It depends on exactly what they want the cable for.
12 or more core cable in 1.5mm2 cross section is most certainly available....and very nice cable too

There are all sorts of things people run in trailers using 12 pin plugs, work lights, rotating beacons, battery supplies for a whole variety of reasons and with led lighting lots of things now become viable on light cable

If ya looking at running high demand circuits onto a trailer the 12 pin plug is simply not up to it, so why even worry about a cable for that application.

To put it bluntly the 7 pin and 12 pin rectangular plastic plugs are just rubbish any way, they are simply a shocking design in so many ways.

SO the question was...is 12 core cable available....and it most certainly is.

cheers

Spot on,wich is what olcoolone said, make a loom.