If I can chime in here...
When it comes to splicing into any of my truck's OEM wiring I am very particular about how I do it. My goal when doing such a job is that I do it in such a way that I can do it once and forget about it.
To that end, I cut the loom wire and bare about 8-10mm at each end, slip on a short length of glue lined heat shrink, bare the wire that will be joined to the loom wire (about 20mm). I then twist that over the two cut ends of the OEM wire, ensuring that I maintain the original length of the OEM wire (makes it easier and neater if you have to re-wrap the loom with tape). To finish up, the wires are soldered together then I slide the heat shrink over the joint and use a heat gun to finish the job.
This is a much more tedious and time consuming way of doing the splice, compared to using crimp connectors, but I have never had one of these joints fail on me, which is the reason I continue to do it this way.
Don't get me wrong here, I do not think that using good crimp connectors is bad, I just prefer to have soldered joints in this type of splice. Each to their own, I guess...
