when i was in the map Backlot you know when you go up the stairs and you can go up anymore.Well you jump on the rail and you jumpp on it on to the window lay down and scoot over when you get out crouch when you cant walk on edge anymore scoot down 1 INCH!
the keep walking and go on the building IF YOU DONT UNDERSTAND WATCH THIS TUTORIAL VIDEO THANKS TO WHOEVER POSTED THE VID
https://www.youtube.com/watch?v=RywxIBhjjG0