Soldering- Yes you should

You could certainly get by without ever having to solder a single connection. But don't think that you're going to do every electrical job on a moving vehicle that sees extreme vibration, exposure to water and other corrosives and  huge temperature swings with crimp connectors. Just like some parts can safely be bolted together, and others must be welded, you will need to solder as long as the wiring is still made out of copper :)

For basic inside work like radio installations and gauge clusters, most of the connections can be made with a skillfully applied crimp connector. Save a lot of guessing an re-work by having the best possible crimpers. But  as soon as you have a connection that will be exposed to mechanical strain or the environment, it's time to break out the soldering iron. I also wouldn't crimp anything bigger than a 10 gauge wire unless it's a dire emergency or field repair ! And crimping any connection that will be buried in a harness or under a trim panel is just asking for future problems.

I'd be willing to bet that every home in America has on of these little guys- a "pencil iron" that can solder a wire about the size of thin spaghetti with ease. The problem is that this type of iron will be pretty slow for automotive work, it really can't heat up a wire fast enough to get anything serious done. Think about it- once you twist two clean wires together, you need to get the wires hot enough so that you can feed the solder into the junction and have it melt quickly and evenly. The wires need to be hot so that the solder will flow into all of the strands and form a good electrical as well as mechanical bond.

Of course, heavier wires will need a LOT more heat to get the solder flowing nicely, and there's no other way to get slightly corroded wires to co-operate.  Here's my weapon of choice (at a great price!)- for a couple of reasons. This unit is dual heat and warms up QUICKLY- how about 5 seconds. This makes it super simple to solder multiple smaller connections, and controllable enough to add a dab of solder quickly to delicate crimped connectors without melting the jacket.  And the hi temp will make fast work of  the heavier wire in a typical remote start or amplifier installation. This gun will pay for itself in time saved alone, think about making a soldered joint as quickly as using a crimp connection!  And it has an easily replaceable element so it should last for a long time- but note that you can't drop it right on the element, that will most likely break the ceramic core.

Outside of the protected cabin area, only soldering will do. And you should get in the habit of having some heat shrink tubing around to secure the finished connection. I would quickly tape the junction first to give some additional protection, but only heat shrink will withstand the elements. If you're connection to older wiring with any corrosion, you'll need a little  flux to get things going- and plenty of heat. Just remember that you're heating the wires until the solder flows in, NOT the solder itself. If you get in to making battery cable ends, you'll need a little torch to get enough heat going- even th secret weapon can't do that job !