onkey help

[bcode]

I am completely new to javascript but I am trying it works semi-good I added onKeyUp to my forms input and it works. I now need it when you press backspace to go delete and go to the previous input but I have onkeyup I started trying to get it to work with the second script in the code but I don't really know what I am doing. If you need more info let me know I really don't know how to ask this  thank you for any help

<script type="text/javascript">
var active_color = '#993300';
var inactive_color = '#993300';

window.onload = formDefaultValues;

function formDefaultValues() {
  var fields = getElementsByClassName(document, "input", "default-value");
  if (!fields) {
    return;
  }
  var default_values = new Array();
  for (var i = 0; i < fields.length; i++) {
    fields[i].style.color = inactive_color;
    if (!default_values[fields[i].id]) {
      default_values[fields[i].id] = fields[i].value;
    }
    fields[i].onfocus = function() { 
      if (this.value == default_values[this.id]) {
        this.value = '';
        this.style.color = active_color;
      }
      this.onblur = function() {
        if (this.value == '') {
          this.style.color = inactive_color;
          this.value = default_values[this.id];
        }
      }
    }
  }
}

function getElementsByClassName(oElm, strTagName, strClassName){
  var arrElements = (strTagName == "*" && oElm.all)? oElm.all : oElm.getElementsByTagName(strTagName);
  var arrReturnElements = new Array();
  strClassName = strClassName.replace(/\-/g, "\\-");
  var oRegExp = new RegExp("(^|\\s)" + strClassName + "(\\s|$)");
  var oElement;
  for (var i = 0; i < arrElements.length; i++) {
    oElement = arrElements[i];
    if (oRegExp.test(oElement.className)) {
      arrReturnElements.push(oElement);
    }
  }
  return (arrReturnElements);
}
</script>
<script type='text/javascript'>
var n=0
function x(){
if(event.keyCode==35){
n+=1
if (n==2)alert('2 x #')
}

}
</script>

<form action="" method="get">
<input id="1" name="zipcode1" onKeyUp="document.getElementById('2').focus()" onkeypress='x(event)' type="text" value="?" size="1" maxlength="1" class="default-value" />
<input id="2" name="zipcode2" onKeyUp="document.getElementById('3').focus()" onkeypress='x(event)' type="text" value="0" size="1" maxlength="1" class="default-value" />
<input id="3" name="zipcode3" onKeyUp="document.getElementById('4').focus()" type="text" value="0" size="1" maxlength="1" class="default-value" />
<input id="4" name="zipcode4" onKeyUp="document.getElementById('5').focus()" type="text" value="0" size="1" maxlength="1" class="default-value" />
<input id="5" name="zipcode5" type="text" value="0" size="1" maxlength="1" class="default-value" />
</form>

[bcode]

here is a working sample

 

http://code2pixels.com/testzip.html

 

i need to be able to go backspaces