I used to work as an electronics technician.

To do some wiring that'll be bulletproof and last:

1. get wiring rated for under-the-hood heat (the wiring sold at auto parts stores is no good for that)

2. get crimp-on connectors

3. cut the plastic off the crimp-ons

4. put heat shrink tubing on the wire, well away from the end

5. crimp the connector on

6. solder the crimp joint using a thermostat controlled soldering iron

7. move the heat shrink tubing over the joint, and heat it with a bic cigarette lighter to shrink it on

8. voila!

P.S. Crimped connections don't last. After about a year, they'll work loose a bit from vibration, and corrosion will creep in, and you'll get a loose connection that is very frustrating to find. Soldering it prevents that from happening.