Do your 12v electrical work properly.

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.

These things are marketed as the saviour of wiring for people who like to do their own modifications. The idea is simple. If you want to splice two wires together, and one of them is an existing wire (like a +12v line, you can use one of these connectors. One side snaps over the existing wire, and the new wire goes into the other side. When you crimp the metal piece it pierces the insulation on both wires and makes an electrical connection. Clip the plastic snap closed and you're done, right? Well - for now. But later on, that connection will fail in all manner of creative ways and you'll be left with nothing but trouble which, for the most part, will be very hard to track down. Scotchlock connectors need to burn in hell.
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.

The attraction of Scotchlock connectors is that you don't need to cut the vehicle's original wiring to splice lines together to make a Y-connection. In reality you are cutting the wiring though, because of the metal blade inside the connector. So given that, just cut the wiring and do it properly. Take one end of the cut wiring and the new wire, and crimp them together into one end of the bullet connector, and take the other end of the cut wiring and crimp it into the other connector. Simple - that's a Y-connection. Clip the two connectors together and you're done. For connections that don't need to be taken apart again, you can use an even simpler crimp connector and just stuff two wires in one end and one in the other.

It goes without saying that any work you do on the electrical system is best done with the battery disconnected. Although if you're the type that uses scotchlock connectors, chances are you never figured out to disconnect the battery in the first place.