Link tracking with Javascript help needed

Hello,

I am trying get some javascript code to track links that are marked as rel=external.

The javascript will be in a tag on my site but so far I can get it it to work.

Can anyone out there tell me what I am doing wrong from in the function below?

function addEvent(elm, evType, fn, useCapture)
                    {
                      if (elm.addEventListener){
                          elm.addEventListener(evType, fn, useCapture);
                          return true;
                      } else if (elm.attachEvent){
                          var r = elm.attachEvent("on"+evType, fn);
                          return r;
                      } else {
                          alert("Handler could not be removed");
                      }
                    }
                    function externalLinks() {
                     if (!document.getElementsByTagName) return;
                     var anchors = document.getElementsByTagName("a");
                                 var newwindows =0;
                     for (var i=0; i<anchors.length; i++) {
                       var anchor = anchors[i].onmousedown = track;
                       if (anchor.getAttribute("href") && anchor.getAttribute("rel") == "external") {
                                anchor.setAttribute("target","_blank");
                                                        //newwindows++;
                                        }

function track() {
preload_exit = new Image(0,0);
preload_exit.src="http://trackersite.com/e.jpg?ref="+url;

}

                     }
                    }

                    addEvent(window, "load", externalLinks);

Code (markup):

 

I think the problem is that addEvent requires four parameters but you have provided only three.
addEvent(elm, evType, fn, useCapture)
addEvent(window, "load", externalLinks);