If the agent who fitted the box was acting on behalf of the insurer (i.e. paid by them) then you have rights under the law of bailment. They (the agent) has a legal duty to keep your vehicle free from damage while in their control. Should it be damaged in their possession, they are liable for the cost of rectification. You also have different legal rights under the 2015 Consumer Act, again giving rights to sue the agent who fitted the unit for their negligence. You would have to prove their negligence caused the faults you have advised of. The starting point is to complain to the insurer, then take to the Ombudsman, the better way is to sue them via the courts.