jQuery datatables ajax callback

I am using jQuery DataTables and doing server-side data. I'm trying to call a function when the ajax call returns. I tried inserting this fnCallback2 which calls my function and the original function, but jQuery just throws an error (and doesn't tell me what the error is) and skips out.

$("#brands").dataTable( {
"bServerSide" : true,
"sAjaxSource" : "ajax.php",
"fnServerData" : function(sSource, aoData, fnCallback) {
    fnCallback2 = function(a,b,c){
        fnCallback.call(a,b,c);
        update_editable();
    };
    $.ajax( {
        "dataType" : 'json',
        "type" : "POST",
        "url" : sSource,
        "data" : aoData,
        "success" : fnCallback2
    });}});

I also tried adding the fnInitComplete parameter, but that only gets called the first time, not after subsequent pages.

"fnInitComplete": function(){
update_editable();
},

How do I correctly call my code after the ajax request so that the original callback gets called as well?