Unless your vehicle was manufactured before 1st January 1971, the maximum height for a brake light is 1,500 mm, except where the vehicle's structure makes this impractical, in which case a maximum height of 2,100 mm is permissible. Given that one could retro-fit a high-level brake-light just
above the
bottom of the window-glass, positioning it just
below the
top of the window glass might be difficult to justify!
Road Vehicles Lighting Regulations 1989
https://www.legislation.gov.uk/uksi/1989/1796/schedule/12/made
Can you see anything out of that rear window!?! If not, you might find yourself falling foul of the Road Vehicles (Construction & Use) Regulations 1986!
https://www.legislation.gov.uk/uksi/1986/1078/contents/made
It is all
TOO EASY to fall foul of regulations (punishable by fines & penalty points at the least!), when one starts modifying vehicles!!!