For long term use in cars & trucks I have found standard crimp terminals to be a constant fail point. There's 3 options to solve this problem. 1.) Chose a crimp style the incorporates an insulation crimp as well. Like what the different car manufactures use. They are more expensive though. Both for the terminals, and the tool. 2.) Get none insulated crimp terminals. Then use a small amount of solder one the crimp connection. The use of a good quality marine grade heat shrink tubing will be needed as well. This not only keeps moisture out, but by using the appropriate length of heat shrink it will serve as a sufficient strain relief against vibrations. 3.) Learn to solder correctly. Don't over use the solder. Just enough to make the connection. This prevents the solder from wicking 2 inches up the wire. Proper heat control is needed. To little heat, and you get a cold joint with voids filled with acidic Flux. To much heat, and you oxidize the wire & connector. The right amount of heat will drive out the Flux, and leave no voids for unseen corrosion to occur. Oxidized wire and connectors corode quickly. You will need to use heat shrink tubing with this method as well.
With both #2 & #3 if I feel a little extra strain relief is needed, after shrinking the tubing I will cut a piece of the next size up of heat shrink tubing cut 1/4 to 1/2 inch longer. Then shrink it over the top of it. This doubles the thickness, but adds a small section at the end that is more flexible. Since it's just a single layer at that point.
Most solder joint failures in cars, and in the aerospace industry (even higher vibrations) is caused by oxidation (to much heat or heated to long) or by Flux filled voids (not enough heat to drive out the flux). The application of far to much solder can cause failures as well. As all that excess solder will wick up the wire creating a strain point that can fail over time. The solution is to practice on a few pieces of wire until you get the hang of it.
There's a reason why the aerospace industry solders the connections on critical connections. When done correctly it's far superior over time. Not that they don't use crimp connections. It's just that thier crimp connectors are different, and require different tools to make the crimps safe to use in an airplane. Standard crimps lossen from vibration, or the wire breaks at the end of the connector. The nylon insulation that extends past the crimp provides little to no strain relief. If a standard crimp connector must be used in a car. Do your self a favor and use a piece of heat shrink tubing that extends past the nylon portion 1/4 to 1/2 an inch after being shrunk. This seals the connection preventing corrosion, and provides the needed strain relief. It also prevents the wire from being pulled out of the crimp if it gets yanked. Specialy with butt connectors.