Sorry for asking the question, I found the answer myself in a swedish magazine... On all cars with an gasoline tanc behind the backseat(as MB Sedan and Audi Sedan etc) it is allways difficult to mount/build something good. The best thing in a regular car is a sealed, ported or single-reflex bandpass box directly under the hat shelf.

But with the gasoline tanc there it is a bit more difficult. According to the magazine build a (ported) 4:th or 6:th bandpass box and let it play through the hat shelf. I will try to build myself a 6:th, in that way I can build a box, place it on the right or left side(and use the space there for it) and then mount the tube from the box so it goes through the hat shelf. I do have to make the existing hole(where the orginal speakers would be mounted) slighty bigger to give room for a single 3" tube.

The speaker in a 6:th is inside the box so I don't have to make it to play through the hat shelf aswell=I can take advantage of the left or right trunk side for the box.

If a remember I will post a note here how it went.

Regards /Patrik from Sweden