I do a simple lap solder, and cover it with heat-shrink. It's more than sufficient for this task. I tin both ends of the wires, trim to about 1/4" exposed tinned end, slip the heatshrink over one side, and then just hold them together and apply heat. Makes a slim joint that doesn't impede getting the whole mess back in the shell.
I try to leave a little slack in the locomotive wiring so the next time I replace the regulator, I can use the same process. Normally, I cut the regulator leads to a couple of inches.