Since were on topic, may I ask what solder works best with this particular type of wire? I've periodically had issues soldering this old wiring so thought I'd see what say ya all been using?
Pretty much any decent quality flux cored solder will do what you need for automotive wiring. The issue you are probably up against trying to solder old wire is contamination. If the copper has blackened with corrosion/age then as you heat it the corrosion contaminates the joint. Ideally you need to cut back to clean copper (the same is true with a crimped joint as the corrosion will continue and create resistance between the wire and the crimp connector) the other thing is heat the wire not the solder, if you hold the heat under the wire and apply the solder from the top then if it's hot enough the solder will melt on the wire and be drawn into the joint.
If you can't cut back to clean copper then heat the wire and apply plumbers flux, heat until it's bubbling and wipe off with a rag, repeat a couple of times then apply the solder and you have half a chance of a decent joint. (Be careful not to overheat and melt the insulation)