i used to have a 96 328i with 2 10" subs
is urs the newer gen ones where the rear seats fold down?
and how big is ur sub box. ? 1 sub or 2 sub. ?
bcuz i had the ported 2 10" sub box.. the subs fit perfectly between the shock towers in the trunk . so i didnt have any problem for the sub to move around..
plus subwoofers are usually really heavy and box is carpetted so it usually does not move around at all.