I really try to NOT solder anything in the car these days. Sometimes you have to, but it is better if you don't. Vibration, corrosion and metal fatigue are the reasons.
I've stripped Subaru harnesses a couple times. They crimp everything. Under all the insulation and tape, there are MANY butt splices and ground crimps for a pigtail here and there. None of them were bad, they all were strong and not corroded.
I also try to never use tape, preferring heat shrink as much as possible instead. But this requires a lot of planning ahead with respect to wire length, room to slide the heat shrink, and sometimes staggering connections so the whole bundle doesn't get too fat.
When I do solder, I use marine heat shrink(mostly from Harbor Freight), which has a clear goo coating inside that seals around the wire insulation when heated. I also support the splice on both ends to something solid so vibration can't happen. Pretty much the only place I solder is to butt splice wires. And I really can't stress the support thing. If you immobilize the joint, it goes a long way to making it survive.
I have seen MANY of those solder/heat shrink combo connectors fail that Alan talks about. In my mind, they are junk. I have no idea what solder is used inside, but it appears to be a low enough temperature that they always corrode. I don't think any moisture gets driven out, hence they corrode and fracture. I know people use them with seeming success, but let us see how they fare in 5 years.
Harbor freight sells 4 foot lengths of multiple diameters of heat shrink for a low, low price. It comes in 6 or 7 little white boxes that are clear shrink-wrapped together. These are easily overlooked on the shelf in the store. This is what I used to make my EFI harness. Each box contains a 4 foot coil in a different size. Handy, for sure. I cut pieces of 18 or 24 inches. More than that makes it impossible to slide the wires through. I used a piece of MiG wire as a fish/pull. Overlap each joint by an inch, letting the first one cool before attempting to overlap the next one.
My best purchase ever for wiring was the three slot ratchet crimper on Amazon for 30 bucks or so, rather than the several hundred dollar Klein or the like. This crimper handled the large GM Weatherpack crimps of 14-16 gauge to the tiny ends on my ECU harness of 22-24-26 gauge. Practice first, and making strong and reliable crimps becomes easy.
I also purchased some 2 and 3 conductor connector pairs. Male and female plastic ends, silicon inserts, and metal crimps. These are pretty inexpensive Deutsch connector knock-offs, say less than 10 bucks for a dozen pairs. I used these on some aux gauges in my Cayman, replacing the horrid bullet connectors that the gauge came with. I also had to use these on the headlight wiring of my truck. The previous owner cut out the connectors that were there, and used those cheesy heat shrink/solder combos. Which failed, of course, even though no water sprays on them, they are behind the fender liner.
I picked up spade connectors(think Bosch blue coil) and ring connectors from Ace Hardware. These have nylon insulation and appear to be the same quality as Wurth. I always back up these crimps with heat shrink as a stress relief from the crimped area to the wire insulation.
And finally ALWAYS use grommets when passing through a bulkhead.
Plan ahead, use the right tools, and wiring is easy. Especially today, the cost of the right tools is not high. Maybe I'm a weirdo, but I enjoy this stuff.
Cheers.
