Admittedly, I know little to nothing about car bombs or assassination in general. The closest I ever came to blowing up a car was when I was working on my MG. The battery was behind the passenger seat just in front of the gas tank and fuel pump. I managed to spark the battery really big...while disconnecting it in order to replace the leaky fuel pump. Fun times.

I would imagine that he'd do the bulk of the work while the car was in the garage (the stone hut in the novel), then have a way to activate it when he parked it out in the courtyard. Since Fabrizzio was to retrieve the car from the garage, he's already where he needs to be. When he parks up in the courtyard, he can quickly connect the wires to the ignition switch and get out.