Just Fitted Already a problem

The technician arrived at 7:30 to install a unit in my new Nissan. After spending 2 and a half hours he told me the unit is fitted. I made a quick spot check and everything seemed fine. I then needed to go out. I noticed the light on my hand brake was not working anymore. I phoned the technician who came back. After another hour he told me he cannot fix it. It was definitely working before he came. I want the unit removed please