"Use the source, Luke"
In function COM_siteHeader in lib-common.php, you'll find:
Text Formatted Code
// If the theme implemented this for us then call their version instead.
$function = $_CONF['theme'] . '_siteHeader';
if( function_exists( $function ))
{
return $function( $what );
}
And yes, your function should go into your theme's functions.php
bye, Dirk