Too many posts!
That's all really interesting. I've used a lot of cheaper crimped connections over the years, in ignorance of better ones being out there, and have had the occasional failure for sure. Ditto for soldered joints.You need to get some serious crimping tools. Jaycar, Altronics, etc don't have them. Look on RS components and you'll see they are 400$ and up. That is what the factory uses to get those superb crimps you see. No, they don't use that they use a machine with hydraulic actuation, but the dies doing the actual crimping are probably the same or similar.
Get yourself a Deutsch crimper (from Deutsch or Knipex) they are the cheapest standard of proper crimper. I think I paid just under 300 for mine, but like I said, if you want anything else they are above 400$. Deutsch is good enough for the mining industry so I call that good. Plus you can get the terminals by themselves (see RS Components) in any size you want (check what sizes your crimper will do) and the connectors (housings - plugs and sockets) too. In all sorts of versions, all water proof plus panel mounts, brackets, goosenecks, boots, right angle, straight, flush etc. you name it. They are not cheap but they are forever reliable.
I used to crimp and solder Deutsch , the soldering is contained inside the terminal, the wire has no solder on it past the terminal and the silicone seal serves as the support so if the wire is wiggled and moves, it only moves up to the silicone seal, not behind it where the soldered joint is. I have also used large glue lined heatshrink tube over the plastic casing just to make double sure no ingress of any kind is possible including petroleum derived chemicals of any kind. Works. In recent times I have started to trust Deutsch enough that I don't solder anymore, just crimp and it's done. I also use silicone wire, very expensive but practically impervious to anything except for knife. Won't even burn or melt, so ideal for engine bay wires. For the current they are capable of, size is also better than vinyl insulated. A silicone wire crimped with the correct tool and with sealed Deutsch connectors at both ends will not let you down.
Crimping is not that easy for the home gamer. As I mentioned above, you enter a painful world with cheap crimpers (I have an entire stash of junk) or splash out on something good, but that comes with its own problems. Every car manufacturer has their own terminals, plugs, sockets and it never ends! There's more standards than digital camera .RAW file formats!! Which one to get?! And will you then change all the connectors on the car to that standard? And will you find the terminals?
That is why I chose Deutsch. It's universal, sealed, well represented everywhere, parts readily available, relatively cheap and original crimpers are not a fortune to buy. They are easy to take apart or pull individual wires with terminals on out and put back in with no special tools. Extremely convenient if you know what it means to try and take the wires out of a Toyota or Honda plug. Plus their run of the mill contacts are silver nickel or you can get the top of the line gold ones! Yeah, baby! Don't tell your wife the car is getting gold terminals though.
Which is where soldering comes in. Solder joints are fragile at the transition point between the solder and the bare wire. Copper work hardens easily so if there is a preferred flex point where it will always flex, that point will work harden and break. Soldering creates one such point. The soldered part is stiff and doesn't flex, the bare wire without insulation is now the preferred flex point. Use heatshrink, generously extending either side of the solder joint and you won't have problems.
I used to use a NASA standard solder joint (see here: https://i0.wp.com/cdn.makezine.com/uploads/2012/02/western-union-or-linemans-splice.jpg?resize=514,580) with heat shrink tube. Didn't know it was called that, I came up with it out of despair. Don't worry about the picture being with single wire, it works very well on multistrand.
Silicone wire is probably the best (but the most expensive, of course) here too. It uses so many strands of copper and it is so flexible (ever heard of "wet noodle"?), it is difficult to work harden any particular strand of wire in it simply because there's so may of them and they're so fine. It is therefore unlikely you're going to bend the same wire the same way twice (which is how work hardening happens). It does happen eventually but it will take a long time. If you use heatshrink you will get a joint that will outlast the car and most likely you too.
And if you have solder wick past where you want it to, your technique is not great. I use a high power iron with large chisel tip. I hold it inverted in a stand. Put some solder on it to create a small puddle like a blob and then come in with the bare wire and insert it in the blob as far in as you want it tinned and pull it out. Be quick, not lighting fast but don't dwell. You need to be in and out before the flux loses its efficiency. There will be no excess tin, no wicking no nada. If you have excess tin, refresh the blob, come in with the wire again, the excess will be retained by the blob. If you have sharp spikes you're too slow (left the blob dwelling too long and the flux has evaporated) and/or the iron is not hot enough. Practice.
After that, you can trim it back if needed to how long you want it tinned if you overshot.
And you can crimp it at whatever point you want to crimp it. You can crimp it so the tinned portion doesn't project out of the terminal. And you can add solder to the join after crimping. Again, because the wire is tinned already you can do it in a fraction of a second (tin heats quickly and spreads the heat evenly) and there will be no wicking because the rest of the wire is not hot enough yet to suck the tin along.
I know it's hard when you have two wires barely reaching each other poking out of holes at opposite ends of the dash in the footwell, but believe me, it is possible.
And if it's not possible, just put a connector in line. It will be easier to service!
As for the original post gizmo, I gave up on any auto terminals, just too much trouble. When something needs fixing, it's converted to Deutsch. There are OEM senders, etc that come with their own (generally Bosch) connectors moulded as part of the casing, so I keep a selection of Bosch sockets/plugs for those situations. Bosch connectors are not bad either, so I don't have a problem with those. There are also connectors moulded into globe holders in indicators, headlights and such. If I can solder pigtails I can terminate with a Deutsch plug and pot the soldered end in situ I do that. If not, I don't yet have a solution.
So the Deutsch system looks interesting. I can find "Deutsch" crimping tools on line for prices that range from $44 to $860 however. Plenty of manufacturers offer them. Are the less expensive ones any good - they look the same so use the same crimping system I guess. How to select please?