Follow the K.I.S.S. rule!!! Keep It Simple, Stupid - Always start with the simplest things first. Always check fuses(don't forget that most after market radios have fuse(s) in it wiring harness), connections, light bulbs, and other basic things. If you have multiple items not working, see if they all have something in common, then check that.
Blown Fuses - Radio won't turn on? All of your dashboard lights went out? Check all of the fuses and relays. Most vehicles now have 2 fuse blocks. One in the passenger compartment and one under the hood. If you replace the fuse and it blows immediatly, then you have a short. A short is where the power is going to either ground or another circuit. If it turns out that you have a short, it can be real easy or real hard to find. Check the both ends of the wire. It could be as simple as a loose wire. Or, it could be that the insulation has been remove from a section of wire and that wire is now rubbing against metal. This is more common underneath the vehicle and in the engine compartment.
No Power or signal - If something doesn't come on when you turn it on (i.e. your radio) and the fuse isn't blown, you most likely have an open circuit. This means that the path of the power or signal have been interuppted-basically the wire has been cut or the connector isn't hooked up right or is loose. This is were a multimeter can come in handy expecially if you have extra long leads on it. I happen to have a really nice Fluke with 8 feet leads on it so I can stretch 16 feet. Set it to the resistence setting( make sure that the battery is disconnected-power will blow a multimeter in the resistence mode). put one test leads at the beginning of the wire and the other lead on the other end. You should come up with a reading very close to 0 or a little higher depeding on what all is attatched to the wire. If you come up with infinite resistense there is an open somewhere. Check the connections, then trace the wire. You'll most likely find that the wire has broken or that a fused link needs to be replaced.
Battery is dead after vehicle sits over night - This usually happens when you have a short. The best way to test if you have a battery draining short is to disconnect the negative cable from the battery. Attach the clip of a test probe light to the negative post of the battery. Touch the light to the negative cable. If it lights up, then you have a short. To figure what drawing the power, hook the light up as before, and pull one fuse at a time until the light goes out. Once you fimd which fuse it is, that'll narrow it down to a couple of components. Then just follow the procedures above.
Shooting
