If you've ever dabbled in 12v electrics in your car, you've probably come across one of the enemies of electrical systems - the spiteful little piece of metal and plastic known as the scotchlock connector.
If you're going to do your own electrical work in your car, do it properly. Use bullet connectors or crimp connectors if you're cutting and splicing wiring. They're just as easy to use as scotchlocks but they're a thousand times more reliable. You need to invest in a crimp tool - looks like a pair of pliers with notches in the jaws - because pliers won't crimp the electrical connection properly. But the end result is secure wiring that won't vibrate or pull apart and won't create electrical problems further down the line. Everything from engines that won't start, to flickering lights, to fried engine management computers to non-working instruments. I've seen all these things caused by sketchy wiring from people trying to scotchlock a new radio into their car.