var x_maplinks = document.getElementById('ctl00_ContentBody_MapLinks_MapLinks');
var x_ml_li = document.createElement('li');
var x_ml_a = document.createElement('a');
x_ml_a.setAttribute('href',
  'https://www.opencaching.de/map2.php?mode=fullscreen&zoom=15&lat=' +
  window.wrappedJSObject.mapLatLng.lat + '&lon=' +
  window.wrappedJSObject.mapLatLng.lng);
x_ml_a.innerHTML = 'OpenCaching.DE-Karte';
x_ml_li.appendChild(x_ml_a);
x_maplinks.appendChild(x_ml_li);

I just put that into the modifyCacheDetails() function.