this is how i fixed it.
open boot
pull back carpet trim on left hand side of boot wall
locate big black plug with approx 10 wires leading to it
undo plug by pressing little release button
spray well with wd40
put plug back together, making sure it "clicks" into position
should now be fixed
on mine this did the job. i took it apart 3 or 4 times and no joy. on the last try i noticed the "click" and jobs a good un. i suppose its just the plug not sitting properly together and loosening with driving.
good luck
kieron
November 2012