how can I fix my car heater/demister. vauxhall corsa?

iMy car has been in garage 3 times with same problem, and within a day of fixing heater/demister stops again. I've been told it is a wiring problem. Sometimes it will come back on if I go over bump on road or if I gve a sharp thump at the side of the dash board but not always. It is 07 Corsa cloub with 34000 miles on clock.