crash2
-Oh no I picked a side-
That is a good method but it won't work unless you GET RID OF THE FLUX CORE SOLDER! Chances are that crap is for copper ie plumbing you need solid core or solder pellets made for the application. Napa should have pellets on the shelf. Also they make a paste flux that you wipe on the wire, warm with a torch and touch the solder to it and it'll literally suck the solder into the wire. My method on big stuff like that is to drop in a pellet or a wad of solder into the the connector, coat the wire with paste flux then heat the connector till the solder is melted and slowly stick the wire in there to suck up the solder as it slides in.
Really--I use rosin core solder--have for years and I have done dozens and dozens of cables for folks--zero failures...