To check the brake light you have mate find out which wires are + and - and attach it to the battery (you may have to extend the wires) and if it lights up its good and if it doesn't light up its fucked.
But if it lights up connected to the battery and not when connected to the wiring on the boot it may be the wiring.
It'll either be the wiring from bootlid back (the wires sometimes break where they bend as the bootlid opens and closes) or check the wiring the bodyshop made for you (get a multimeter or a 12volt test light on the wiring and get someone to push the brake pedal while you check for power around 12volts or the test light to light up - if youconnect the multimeter to the wrong wires you'll just get a minus voltage reading on the display or the test light wont light up)
Or even the brakelight switch beside the pedal, the way to check the brake light switch is to get someone to push brake pedal and seeif the normal brake lights work (you can doit on your own just reverse upto a car or wall and check in your rear view mirrorthere working)
Its just a case of process of elimination
Outta curiousity how much was the new brakelight ?