Assuming that the battery is known to be good, then the next item to check is the battery terminals - the actual points at which the battery is connected into the electrical system. It is very important that these connections are clean and tight. This simple thing is often overlooked as a source of trouble, and is also a frequent cause of such trouble. Visually inspect the terminals, and clean andor tighten them if needed. To clean, use a small, stiff wire brush or a terminal cleaner which is made for the purpose.
You can buy these at nearly any auto supply store, and most hardware stores. You will also need an appropriate sized wrench with which to remove the battery terminals. Important: Always disconnect the negative (-) battery terminal first, and reconnect it last. This will help to keep from shorting the battery while connecting or disconnecting the terminals and possibly causing damage andor personal injury.
Fixing Wiring Problems: Find an electrical schematic of your vehicle. This can be found either from information in your owner's manual or on the Internet. Trace the wiring from the fuse panel to the switch. You should refer to your electrical schematic for help. You want to make sure that there are no breaks in the connection from the fuse box to the switch. If you did not find a schematic, it might be easier to trace the wiring from the switch back to the fuse box to look for any breaks in the connection.
If you need to replace your auto battery, always test it before you replace it. Also test the entire electrical system of your car. The reason for your battery problem might be a weak alternator or a drain on the system and if this is the case it won't take many weeks before your have to replace the new one as well. What seems to be a battery problem is not always caused by the battery itself. You can acquire more information about how to repair your car's electrical system on the web.