It’s been too long since I’ve played this game but I’ll try my best to help 
I’ve found the following info in an old walkthru, maybe it will help?
Can’t remember is you need the grapnel to climb is so than the following info isn’t of much help 
Breaking into Ferrari’s House
· Go to where the door is and climb the side of the building.
· Climb to the highest roof.
· Walk to the ledge and climb down to the next roof.
· Jump to the lower level.
· Jump across to Ferrari’s house.
· Sneak to the middle window.
· Try to open the window but the bars are rusted.
· Use the oil.
· Loosen the bars.
· Use your toolkit to open the window.
· Go inside.