3rd stop light installation
 

I have a 300GE / 91 and for safety reasons I am installing a 3rd stop light in the back door window
Didn't get many instructions on how to so I wonder if anyone has an idea or did this before. I reckon the electricity and function connection should be made at the left brake light. Hence my main question is how do I access the brake light from inside of the vehicle and bring it to the top of the back door. Thank you so much folks.