I have always used wire nuts on my trailer connections that are inside and not soldered without any problems.
Just use a properly sized nut and some high quality Scotch #33 electrical
tape. Wrap a few turns of tape
around the wires just below the nut, then up onto the nut and back to the wires with a couple of more turns.
The last two turns are put on firmly but do not stretch the tape here. Cut the tape and put the last couple
of turns on without stretching it. If you really want over kill, you can put a small wire tie around the wires
where the tape wrapping ends. This method has worked well for me without the tape coming unwrapped,
yet allowing the joint to be undone without cutting, crimping, etc.