The stock tank is located under the rear seat for the BMW sedans. The driveshaft runs right through the middle of it. The earlier cars actually have two tanks connected with a hose that runs under the driveshaft. The trunk is on the other side of the rear differential from the tank. It varies somewhat by model, but the nearest part of the trunk is roughly 18" or more away from the stock tank location. Without the exception, the only way to get a cell (of typical capacity) would be to cut out the rear seat area and go up and/or put it in the passenger compartment. I doubt anyone believes those are good ideas.