The feature you looking for is called a
Through-loading system, its a option for the entire 3 series range.
If your car has this, then you need to look in the boot under the parcel shelf. If you find two levers on either side of the shelf (one for the right seat and another for the left) with a seat symbol on it, then Bob's your uncle

Pulling on it should unlock the seats.
Good luck:thumbsup:, its a great feature to have, comes in handy from time to time.