Need connector suggestions
This is to illustrate @WhatRoughBeast's suggestion.



I don't have a 12 AWG wire, so this is a 1:3 scale model.  The bigger wire is 20 AWG.  The smaller wire is 28 AWG.  The butt splice is for 22-18 AWG.

Butt splices for 12-10 AWG are fairly common.  It should be able to accommodate a 12 AWG main wire and a 24 AWG next to it.

For outdoors, I would use an uninsulated butt splice (just the metal tube).  Then slide adhesive-lined heat shrink over it.  When it's heated and shrunk, the adhesive inside melts, fills the voids, and finally cures.  This is an efficient method for keeping the water out.