How do I fix my Nissan Note Fan/ heater that works intermittently?

My 1.5 2007 Nissan Note diesel has a problem with its fan Heater. It works intermittently. I have taken it to the dealer but as it was working when I took it in they could not diagnose and fix it Yet it will cut out and in without warning ( though going over a bump does sometimes cause it to kick back in) Could there be a loose connection and if so how would I get to it and sort it. I am reluctant t have the dealer replace the system if it is something minor