title: TYPO3 own extension Detailpage with form and counter
body: <p>I have a TYPO3 car extension wir the model "car" there is a field of <em>maximale number of cars</em>. 
  On my ShowAction I need a form. When the form is submitted, the number of <em>maximale number of cars</em> should be reduced -1. "</p>
  
  <p>How do I do that? With PowerMail or form? How can I make the reduction -1?</p>
  
  <p>I'm looking forward to any help :-).</p>