BY "these", do you mean the cheap plastic things you buy at any auto parts store? If so, I'm not a fan.
If you need to use that sort of connector, here's hoping there's something better out there.
A good method, if you can't get in there to solder, is to carefully remove 3/8"-1/2" of the insulation of the existing wire, then insert something between the wire strands (assuming not solid core) so about half are on each side. Once you've created an opening, pass the stripped end of the new wire through the opening and twist everything together. You'll figure it out from there.
Then there's these, which I've used a lot over the last few years with great results ...
