No properly repairing the problem

My company has purchased 3 brand new Ford Bantams a couple of years ago. We took out a full maintenance plan for 5 years on all 3 vehicles at R25 000 EXTRA per vehicle. I have had endless problems on every door lock after about 9 months. I have taken the vehicles in each time and they repaired them. Weeks or 2 months pass and the same problems occur. I have relayed my concerns about for permanent solution to fix the problems with the dealership. They provided me with a newly cut key but the problem has now occurred again. There is a clear issue with these door locks. All I expect is for the locks to be replaced and not repaired all the time as this is clearly a problem. I have to therefore pull a bakkie off the road for 2 days before I can get it back. We run a business using the bakkies and this is not only loosing the company money but its wasting our time.