Working on school buses for 33 years, I have installed hundreds of them of all types and quality, most do the job they need to do. For my own stuff, I use uninsulated with shrink sleeve over them. The insulated sleeve type work OK but take up a lot room if you are doing a bundle of wires. You can stagger them out for less mass, but then you have a longer section of stiff wire. For trailer work, the insulated sleeve type are great, not really needed for under dash.
The type Phil shows just above are good choice and won't make a huge bundle when done. If you are good at soldering, just solder the wires together and sleeve over them, this is a good method for really small wires. For bigger wires I will strip the plastic off cheap connectors and use the bare metal with solder with sleeving over them. I seldom crimp any of mine, and if i do its very carefully done so the wire is just held tight enough till I get solder in them. It keeps them straight and round for a better, more professional look.
