Some systems store data in memory that is lost when the battery is disconnected. Do the following steps before disconnecting the battery.
Make sure you have the anti-theft code(s) for the audio and/or the navigation system (if equipped).
If you are replacing the audio unit, write down the audio presets (AM and FM), and the XM audio presets (if equipped), because the audio unit does not retain the presets after the battery is disconnected.
Make sure the ignition switch is in LOCK (0).
Disconnect and isolate the negative cable from the battery.
NOTE: Always disconnect the negative cable from the battery first.
Disconnect the positive cable from the battery.