Thanks coopertaz! I didn't want to touch the roof lining, I'm worried I won't be able to put it back nicely :-D
I went for the right side because of the fuel tank. Like you said, for earth, I used the mounting bolt of the light
For the plus side, I used a T-tap wire connector on the correct wire (green and purple).
Now everything works perfectly, I just need to find a way to hide correctly the wire, because at the moment it's hanging :-/
Oh, and fixing again the back seat wasn't fun at all: the two screws attaching the back and the base are not fun to put back. The back is attached with some metal plates… or I should say, "razor blades" :-D
Thanks hennasxi!
I needed to remove the backseat, because I dodn't have holes accessible without removing it.
I don't know if all MPI are this way, but I have the metallic parcel shelf and it is covered with some "hard carpet" (not sure how to call it). The only holes I had were the one used to screw the back to the parcel shelf. The others holes were used by the speakers.
So I used a hole on the side of the car: the wire go near the rear window straight to the boot (will try to make pics later, even if I replaced the seat).
How did you cleanly route the wire to the side so it doesn't show?