function ident() {
  if(document.getElementById('ident').checked) {
    
	if(document.getElementById('kp_anrede_frau').checked)     document.getElementById('kunde_anrede_frau').checked  = true;
	if(document.getElementById('kp_anrede_herr').checked)     document.getElementById('kunde_anrede_herr').checked  = true;
	if(document.getElementById('kp_anrede_firma').checked)    document.getElementById('kunde_anrede_firma').checked = true;
    
	document.getElementById('kunde_titel').value           = document.getElementById('kp_titel').value;
    document.getElementById('kunde_name').value            = document.getElementById('kp_name').value;
    document.getElementById('kunde_vorname').value         = document.getElementById('kp_vorname').value;
    document.getElementById('kunde_ansprechpartner').value = document.getElementById('kp_ansprechpartner').value;
    document.getElementById('kunde_strasse').value         = document.getElementById('kp_strasse').value;
    document.getElementById('kunde_plz').value             = document.getElementById('kp_plz').value;
    document.getElementById('kunde_ort').value             = document.getElementById('kp_ort').value;
    document.getElementById('kunde_mail').value            = document.getElementById('kp_mail').value;
    document.getElementById('kunde_telefon').value         = document.getElementById('kp_telefon').value;
    document.getElementById('kunde_anmerkung').value       = document.getElementById('kp_anmerkung').value;
		
	document.getElementById('kunde_titel').disabled           = true;
	document.getElementById('kunde_name').disabled            = true;
	document.getElementById('kunde_vorname').disabled         = true;
	document.getElementById('kunde_ansprechpartner').disabled = true;
	document.getElementById('kunde_strasse').disabled         = true;
	document.getElementById('kunde_plz').disabled             = true;
	document.getElementById('kunde_ort').disabled             = true;
	document.getElementById('kunde_mail').disabled            = true;
	document.getElementById('kunde_telefon').disabled         = true;
	document.getElementById('kunde_anmerkung').disabled       = true;
  } else {
    // Formularfelder wieder entsperren
  	document.getElementById('kunde_titel').disabled           = false;
  	document.getElementById('kunde_name').disabled            = false;
  	document.getElementById('kunde_vorname').disabled         = false;
  	document.getElementById('kunde_ansprechpartner').disabled = false;
  	document.getElementById('kunde_strasse').disabled         = false;
  	document.getElementById('kunde_plz').disabled             = false;
  	document.getElementById('kunde_ort').disabled             = false;
  	document.getElementById('kunde_mail').disabled            = false;
  	document.getElementById('kunde_telefon').disabled         = false;
  	document.getElementById('kunde_anmerkung').disabled       = false;
  }  
}

<form action="absenden.php" method="post" enctype="multipart/form-data">

...onClick="ident(); missing_fields()"...

function plz_ereg(ort_id, plz_id, suchstring) {
   
  var wohnort = new Array();

  wohnort[0] = new Object();
  wohnort[0]["plz"] = "1010";
  wohnort[0]["ort"] = "Wien";

  wohnort[1] = new Object();
  wohnort[1]["plz"] = "1020";
  wohnort[1]["ort"] = "Wien";

...

  // Treffer Variable initialisieren
  var counter = "0";

  for (var i = 0; i < wohnort.length; i++) {
    if(wohnort[i]["plz"] == suchstring){ 
      document.getElementById(ort_id).value = wohnort[i]["ort"];
      counter++;
    }
  }
  
  // Prüfen, ob PLZ eingegeben ist, jedoch kein Treffer erzielt wurde -> Wert des Felder Wohnort leeren
  if((document.getElementById(plz_id).value != "") && (counter == "0")) { 
    document.getElementById(ort_id).value = "";
  }
}